add aws_mqtt_has_non_empty_username #1415

Job	Run time
windows-shared-libs	3m 55s
windows-vc17 (x86)	4m 17s
clang-sanitizers (,thread)	3m 30s
windows-vc17 (x64)	4m 20s
downstream	2m 54s
macos (kqueue)	1m 0s
macos (dispatch_queue)	57s
macos-x64	1m 39s
windows-app-verifier	3m 54s
clang-sanitizers (,address,undefined)	3m 29s
windows	4m 15s
linux-compat (al2-x64)	2m 49s
linux-compat (fedora-34-x64)	2m 33s
linux-shared-libs	2m 43s
linux-compat (manylinux2014-x86)	2m 39s
linux-compat (rhel8-x64)	2m 48s
linux-compat (manylinux1-x64)	2m 34s
linux-compat (manylinux1-x86)	2m 48s
linux-compiler-compat (clang-17)	3m 3s
linux-compat (manylinux2014-x64)	2m 47s
linux-compat (opensuse-leap)	2m 48s
linux-compiler-compat (clang-8)	3m 2s
linux-compiler-compat (gcc-4.8)	2m 53s
linux-compiler-compat (clang-11)	3m 2s
linux-compiler-compat (gcc-6)	3m 2s
linux-compiler-compat (clang-15)	3m 3s
linux-compiler-compat (gcc-7)	2m 59s
linux-compiler-compat (gcc-11)	3m 2s
linux-compiler-compat (clang-10)	3m 3s
linux-compiler-compat (clang-9)	3m 2s
linux-compiler-compat (gcc-5)	2m 47s
linux-compiler-compat (clang-6)	3m 2s
linux-compiler-compat (gcc-8)	3m 2s
	1h 37m 41s

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