MINOR: [C++][Compute] Remove resolved TODO comments in case_when null validation #4162

Job	Run time
check-labels / Check labels	25s
JNI ${{ matrix.platform.runs-on }} ${{ matrix.platform.arch }}	1s
JNI macOS	1s
matrix.title	1s
ARM64 Windows 11 MSVC	0s
ODBC	1s
report-extra-cpp	0s
	29s

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