Fix wrong use of mempty deposit in pool registration test #19647

Job	Run time
fourmolu	1s
shellcheck	2m 26s
build (9.10.2, ubuntu-latest)	1s
cabal-format	2m 26s
build (9.6.7, ubuntu-latest)	2m 26s
build (9.12.2, ubuntu-latest)	1s
Check branch history	1s
gen-hie	1s
build (9.8.4, ubuntu-latest)	2m 26s
Check that there are no additions of `undefined`s in the PR diff	1s
Check if formal-ledger-specifications SRP is valid	1s
codegen	1s
${{ matrix.package }}-${{ matrix.ghc }}-${{ matrix.os }}	1s
Tests completed	4s
Send a slack notification on \#ledger-internal if the nightly build failed	1s
	9m 58s

Provide feedback