Fix warning in generator

Fixes
```
Use of uninitialized value in concatenation (.) or string at template qt5-tools/mingw-w64/PKGBUILD.sh.ep line 44.
```

devel/generator/templates/fragments/mingw-w64-qt5-executable_suffix.sh.ep

@@ -1,4 +1,4 @@
-% return undef if $static_variant;
+% return '' if $static_variant;
   # make sure the executables don't conflict with their mingw-qt4 counterpart
   for _arch in ${_architectures}; do
     for exe_file in "${pkgdir}/usr/${_arch}/bin/"*.exe; do