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

src/common/strconv.cpp

@@ -232,12 +232,6 @@ wxMBConv::ToWChar(wchar_t *dst, size_t dstLen,
         if ( !srcEnd )
             dstWritten++;
 
-        if ( !lenChunk )
-        {
-            // nothing left in the input string, conversion succeeded
-            break;
-        }
-
         if ( dst )
         {
             if ( dstWritten > dstLen )