Don't generate filters with empty Includes

Avoids VS warnings:

> smoketest\Library_smoketest.vcxproj.filters : warning  : Unable to
> read the filters file "Library_smoketest.vcxproj.filters". Please
> unload the project "Library_smoketest.vcxproj" to be able to edit it.
> The required attribute "Include" is empty or missing from the element
> <Filter>.

Change-Id: I0f02e837823a5e7a0355fd0f895f00c63a5df6fc
Reviewed-on: https://gerrit.libreoffice.org/51684Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

bin/gbuild-to-ide

@@ -945,7 +945,7 @@ class VisualStudioIntegrationGenerator(IdeIntegrationGenerator):
 
     def get_subfilters(self, proj_filter):
         parts = proj_filter.split('\\')
-        subfilters = set([proj_filter])
+        subfilters = set([proj_filter]) if proj_filter else set()
         for i in range(1, len(parts)):
             subfilters.add('\\'.join(parts[:i]))
         return subfilters