[ThreadNetworkDirectoryServer] Add missing nullptr check for iterator (backport #43619)

[mergify]

Summary

• Missing nullptr check in ReadThreadNetworks, which can occur if we fail to allocate an object from pool (relevant for devices with fixed size pools and not heap)

Testing

Testing in CI, this Cluster is not testable using unit tests yet

---

This is an automatic backport of pull request #43619 done by Mergify.

[github-actions]

PR #72379: Size comparison from 09d0453 to a5e84dd

Full report (48 builds for bl602, bl702, bl702l, cc13x4_26x4, cc32xx, cyw30739, efr32, nrfconnect, psoc6, qpg, stm32, telink, tizen)

platform	target	config	section	09d0453	a5e84dd	change	% change
bl602	lighting-app	bl602+mfd+littlefs+rpc	FLASH	1103512	1103512	0	0.0
			RAM	179034	179034	0	0.0
bl702	lighting-app	bl702+eth	FLASH	656910	656910	0	0.0
			RAM	135001	135001	0	0.0
		bl702+wifi	FLASH	834102	834102	0	0.0
			RAM	124541	124541	0	0.0
		bl706+mfd+rpc+littlefs	FLASH	1066760	1066760	0	0.0
			RAM	117405	117405	0	0.0
bl702l	contact-sensor-app	bl702l+mfd+littlefs	FLASH	897012	897012	0	0.0
			RAM	105716	105716	0	0.0
	lighting-app	bl702l+mfd+littlefs	FLASH	980024	980024	0	0.0
			RAM	109892	109892	0	0.0
cc13x4_26x4	lighting-app	LP_EM_CC1354P10_6	FLASH	764852	764852	0	0.0
			RAM	103376	103376	0	0.0
	lock-ftd	LP_EM_CC1354P10_6	FLASH	776656	776656	0	0.0
			RAM	108560	108560	0	0.0
	pump-app	LP_EM_CC1354P10_6	FLASH	722620	722620	0	0.0
			RAM	96948	96948	0	0.0
	pump-controller-app	LP_EM_CC1354P10_6	FLASH	706912	706912	0	0.0
			RAM	97156	97156	0	0.0
cc32xx	air-purifier	CC3235SF_LAUNCHXL	FLASH	550146	550146	0	0.0
			RAM	205176	205176	0	0.0
	lock	CC3235SF_LAUNCHXL	FLASH	583386	583386	0	0.0
			RAM	205384	205384	0	0.0
cyw30739	light	CYW30739B2-P5-EVK-01	unknown	2040	2040	0	0.0
			FLASH	664281	664281	0	0.0
			RAM	77480	77480	0	0.0
	light-switch	CYW30739B2-P5-EVK-01	unknown	2040	2040	0	0.0
			FLASH	625837	625837	0	0.0
			RAM	73800	73800	0	0.0
	lock	CYW30739B2-P5-EVK-01	unknown	2040	2040	0	0.0
			FLASH	646809	646809	0	0.0
			RAM	76800	76800	0	0.0
	thermostat	CYW30739B2-P5-EVK-01	unknown	2040	2040	0	0.0
			FLASH	621353	621353	0	0.0
			RAM	70904	70904	0	0.0
	light	CYW30739B2-P5-EVK-02	unknown	2040	2040	0	0.0
			FLASH	684125	684125	0	0.0
			RAM	80120	80120	0	0.0
	light-switch	CYW30739B2-P5-EVK-02	unknown	2040	2040	0	0.0
			FLASH	645465	645465	0	0.0
			RAM	76352	76352	0	0.0
	lock	CYW30739B2-P5-EVK-02	unknown	2040	2040	0	0.0
			FLASH	666525	666525	0	0.0
			RAM	79352	79352	0	0.0
	thermostat	CYW30739B2-P5-EVK-02	unknown	2040	2040	0	0.0
			FLASH	641197	641197	0	0.0
			RAM	73536	73536	0	0.0
	light	CYW30739B2-P5-EVK-03	unknown	2040	2040	0	0.0
			FLASH	684125	684125	0	0.0
			RAM	80120	80120	0	0.0
	light-switch	CYW30739B2-P5-EVK-03	unknown	2040	2040	0	0.0
			FLASH	645465	645465	0	0.0
			RAM	76352	76352	0	0.0
	lock	CYW30739B2-P5-EVK-03	unknown	2040	2040	0	0.0
			FLASH	666525	666525	0	0.0
			RAM	79352	79352	0	0.0
	thermostat	CYW30739B2-P5-EVK-03	unknown	2040	2040	0	0.0
			FLASH	641197	641197	0	0.0
			RAM	73536	73536	0	0.0
	light	CYW930739M2EVB-02	unknown	2040	2040	0	0.0
			FLASH	641073	641073	0	0.0
			RAM	72548	72548	0	0.0
efr32	lock-app	BRD4187C	FLASH	948588	948580	-8	-0.0
			RAM	131556	131556	0	0.0
	window-app	BRD4187C	FLASH	1042352	1042344	-8	-0.0
			RAM	127684	127684	0	0.0
	lock-app	BRD4338a	FLASH	746364	746356	-8	-0.0
			RAM	206920	206920	0	0.0
nrfconnect	all-clusters-app	nrf52840dk_nrf52840	FLASH	916480	916480	0	0.0
			RAM	167458	167458	0	0.0
	all-clusters-minimal-app	nrf52840dk_nrf52840	FLASH	859992	859992	0	0.0
			RAM	141065	141065	0	0.0
	all-clusters-app	nrf7002dk_nrf5340_cpuapp	FLASH	914452	914452	0	0.0
			RAM	145676	145676	0	0.0
psoc6	all-clusters	cy8ckit_062s2_43012	FLASH	1674508	1674508	0	0.0
			RAM	212456	212456	0	0.0
	all-clusters-minimal	cy8ckit_062s2_43012	FLASH	1577724	1577724	0	0.0
			RAM	208504	208504	0	0.0
	light	cy8ckit_062s2_43012	FLASH	1450284	1450284	0	0.0
			RAM	197224	197224	0	0.0
	lock	cy8ckit_062s2_43012	FLASH	1482820	1482820	0	0.0
			RAM	224960	224960	0	0.0
qpg	lighting-app	qpg6200+debug	FLASH	745088	745088	0	0.0
			RAM	94228	94228	0	0.0
	lock-app	qpg6200+debug	FLASH	755044	755044	0	0.0
			RAM	94264	94264	0	0.0
stm32	light	STM32WB5MM-DK	FLASH	466512	466512	0	0.0
			RAM	141376	141376	0	0.0
telink	light-app-ota-compress-lzma-shell-factory-data	tl3218x	FLASH	795304	795304	0	0.0
			RAM	44032	44032	0	0.0
	light-switch-app-ota-factory-data	tl3218x_retention	FLASH	724500	724500	0	0.0
			RAM	37024	37024	0	0.0
	bridge-app	tl7218x	FLASH	703582	703582	0	0.0
			RAM	93616	93616	0	0.0
	light-app-ota-shell-factory-data	tl7218x	FLASH	783712	783712	0	0.0
			RAM	100928	100928	0	0.0
	light-switch-app-ota-compress-lzma-factory-data	tl7218x_retention	FLASH	711186	711186	0	0.0
			RAM	54268	54268	0	0.0
	lighting-app-ota-factory-data	tlsr9118bdk40d	FLASH	603866	603866	0	0.0
			RAM	112548	112548	0	0.0
	lighting-app-ota-rpc-factory-data-4mb	tlsr9518adk80d	FLASH	819290	819290	0	0.0
			RAM	99180	99180	0	0.0
	light-switch-app-ota-compress-lzma-shell-factory-data	tlsr9528a	FLASH	747760	747760	0	0.0
			RAM	77424	77424	0	0.0
tizen	all-clusters-app	arm	unknown	5308	5308	0	0.0
			FLASH	1822368	1822368	0	0.0
			RAM	97556	97556	0	0.0
	chip-tool-ubsan	arm	unknown	20700	20700	0	0.0
			FLASH	20976754	20976754	0	0.0
			RAM	9131396	9131396	0	0.0