address review comments and squash the empty string extra earlier

setuptools/dist.py

@@ -89,8 +89,7 @@ def write_pkg_file(self, file):
         )
     if self.provides_extras:
         for extra in self.provides_extras:
-            if extra:
-                file.write('Provides-Extra: %s\n' % extra)
+            file.write('Provides-Extra: %s\n' % extra)
 
 
 # from Python 3.4
@@ -390,7 +389,9 @@ class Distribution(Distribution_parse_config_files, _Distribution):
                 # Since this gets called multiple times at points where the
                 # keys have become 'converted' extras, ensure that we are only
                 # truly adding extras we haven't seen before here.
-                self.metadata.provides_extras.add(extra.split(':')[0])
+                extra = extra.split(':')[0]
+                if extra:
+                    self.metadata.provides_extras.add(extra)
 
         self._convert_extras_requirements()
         self._move_install_requirements_markers()