fix: support empty dictionary keys in input (#245)

* fix: support empty dictionary keys in input

Certain old keys in `msvs_settings` (like "LinkIncremental") are moved
to the top level when translated to `msbuild_settings`.
This change allows to set them when `msbuild_settings` are
authored directly.

* Update pylib/gyp/input.py

Co-authored-by: Christian Clauss <cclauss@me.com>

---------

Co-authored-by: Christian Clauss <cclauss@me.com>

pylib/gyp/input.py

@@ -2536,6 +2536,8 @@ def ProcessListFiltersInDict(name, the_dict):
     lists = []
     del_lists = []
     for key, value in the_dict.items():
+        if not key:
+            continue
         operation = key[-1]
         if operation not in {"!", "/"}:
             continue