Don't end converting prematurely on encountering consecutive NULs.

Don't stop converting subsequent chunks just because the length of one of them
was 0: this can happen if the first character of a string is a NUL or if there
are two (or more) NULs in it later.

Simply remove the check for this and continue as usual even in this case.

Also add a unit test verifying that we do translate NULs in input into NULs in
output.

Closes #16620.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78021 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Disable surrogate-related unit test for MSW.

This test can't work when the in-memory representation is UTF-16, as we can't
convert surrogates to anything else in this case.

This fixes the unit tests broken since r76622, see #16298.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78022 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775