import-checker: increase portability for python 2.6.x

Before this patch, fromlocalfunc() assumes that "module" attribute of
ast.ImportFrom is None for "from . import a", and Python 2.7.x
satisfies this assumption.

On the other hand, with Python 2.6.x, "module" attribute of
ast.ImportFrom is an empty string for "from . import a", and this
causes failure of test-check-module-imports.t.

contrib/import-checker.py

@@ -141,8 +141,8 b' def fromlocalfunc(modulename, localmods)'
     if prefix:
         prefix += '.'
     def fromlocal(name, level=0):
-        # name is None when relative imports are used.
-        if name is None:
+        # name is false value when relative imports are used.
+        if not name:
             # If relative imports are used, level must not be absolute.
             assert level > 0
             candidates = ['.'.join(modulename.split('.')[:-level])]