fix: failing test `Test_null_values` when vim configured without channel #203

Job	Run time
linux (tiny, clang, native)	2m 26s
macos (tiny, macos-12)	1m 11s
windows (HUGE, msvc, no, no, x64, stable)	6m 37s
linux (tiny, gcc, native)	1m 51s
macos (tiny, macos-14)	54s
windows (HUGE, mingw, yes, yes, x86, stable, yes)	12m 14s
linux (normal, clang, native)	5m 36s
macos (normal, macos-12)	6m 36s
windows (HUGE, msvc, no, yes, x86)	9m 16s
linux (normal, gcc, native)	5m 42s
macos (normal, macos-14)	4m 48s
windows (HUGE, mingw, yes, no, x64, yes)	10m 17s
linux (huge, clang, native)	9m 1s
macos (huge, macos-12)	14m 8s
windows (NORMAL, msvc, yes, no, x86)	5m 37s
linux (huge, gcc, native)	10m 37s
macos (huge, macos-14)	13m 59s
linux (tiny, clang, nogui)	1m 55s
windows (NORMAL, mingw, no, yes, x64)	9m 38s
linux (tiny, gcc, nogui)	2m 3s
windows (TINY, msvc, yes, yes, x64)	2m 7s
linux (normal, gcc, i386, ./src/shadow)	5m 53s
windows (TINY, mingw, no, no, x86)	3m 10s
linux (huge, gcc, uchar, testgui, true, dynamic)	9m 40s
linux (huge, clang, asan, 5.1)	16m 43s
linux (huge, gcc, unittests, true)	4m 21s
linux (normal, gcc, vimtags)	2m 8s
	2h 58m 28s

Provide feedback