Bug 1416029: Pass empty TRY_COMMIT_MESSAGE to mozharness on windows. r=dustin

Apparently the window's generic-worker uses batch files for running setting
environment variables. In windows batch files, setting an environment variable
to empty actually unsets. So, in order to avoid mozharness trying to determine
the commit message itself, pass an empty TRY_COMMIT_MESSAGE as a space instead.

MozReview-Commit-ID: 2nJfJE0EaF0

--HG--
extra : rebase_source : 6640ab1037fe48bb9770bcf99cab5f92fbb88c9f

taskcluster/taskgraph/transforms/job/mozharness.py

@@ -221,8 +221,12 @@ def mozharness_on_generic_worker(config, job, taskdesc):
     if run['use-simple-package']:
         env.update({'MOZ_SIMPLE_PACKAGE_NAME': 'target'})
 
+    # The windows generic worker uses batch files to pass environment variables
+    # to commands.  Setting a variable to empty in a batch file unsets, so if
+    # there is no `TRY_COMMIT_MESSAGE`, pass a space instead, so that
+    # mozharness doesn't try to find the commit message on its own.
     if 'try' in config.params['project']:
-        env['TRY_COMMIT_MSG'] = config.params['message']
+        env['TRY_COMMIT_MSG'] = config.params['message'] or 'no commit message'
 
     if not job['attributes']['build_platform'].startswith('win'):
         raise Exception(