Set forall environment variables to empty string if None

If the value obtained is None we now set the variable to
'' instead, in an attempt to make execve() happier about
our 3rd argument, the env dictionary.

Signed-off-by: Shawn O. Pearce <sop@google.com>

subcmds/forall.py

@@ -82,16 +82,21 @@ not redirected.
     rc = 0
     for project in self.GetProjects(args):
       env = dict(os.environ.iteritems())
-      env['REPO_PROJECT'] = project.name
-      env['REPO_PATH'] = project.relpath
-      env['REPO_REMOTE'] = project.remote.name
-      env['REPO_LREV'] = project\
+      def setenv(name, val):
+        if val is None:
+          val = ''
+        env[name] = val
+
+      setenv('REPO_PROJECT', project.name)
+      setenv('REPO_PATH', project.relpath)
+      setenv('REPO_REMOTE', project.remote.name)
+      setenv('REPO_LREV', project\
         .GetRemote(project.remote.name)\
-        .ToLocal(project.revision)
-      env['REPO_RREV'] = project.revision
+        .ToLocal(project.revision))
+      setenv('REPO_RREV', project.revision)
 
       if mirror:
-        env['GIT_DIR'] = project.gitdir
+        setenv('GIT_DIR', project.gitdir)
         cwd = project.gitdir
       else:
         cwd = project.worktree