kconfig: Check empty strings on macro expansion

During macro expansion, the parser accepts bare macros on a line as long
as they resolve to empty strings. However, the current script checks for
whitespace using isspace, which doesn't accurately detect blank strings.

Author: jserv

tools/kconfig/kconfiglib.py

@@ -2623,7 +2623,7 @@ def _parse_assignment(self, s):
             else:
                 break
 
-        if s.isspace():
+        if not s or s.isspace():
             # We also accept a bare macro on a line (e.g.
             # $(warning-if,$(foo),ops)), provided it expands to a blank string
             return