obmc-gerrit: Return empty list on missing MAINTAINERS

Prevents dumping a traceback when no MAINTAINERS is present:

  $ obmc-gerrit push gerrit HEAD:refs/for/master
  Traceback (most recent call last):
    File "/home/andrew/.local/bin/obmc-gerrit", line 63, in <module>
      args.func(args)
    File "/home/andrew/.local/bin/obmc-gerrit", line 51, in do_push
      git.push(args.remote, decorate_refspec(args.refspec),
    File "/home/andrew/.local/bin/obmc-gerrit", line 45, in decorate_refspec
      gargs = gerrit_refspec_args(get_reviewers())
    File "/home/andrew/.local/bin/obmc-gerrit", line 24, in get_reviewers
      with open(mfile, 'r') as mstream:
  FileNotFoundError: [Errno 2] No such file or directory: '/home/andrew/src/openbmc/phosphor-mboxd/MAINTAINERS'

Also, avoid mangling the refspec if the set of reviewers is empty -
there's no need for the trailing '%':

  $ obmc-gerrit push ph-gerrit HEAD:refs/for/master
  ...
  To ssh://gerrit.openbmc-project.xyz:29418/openbmc/phosphor-mboxd
   * [new branch]                HEAD -> refs/for/master%

Change-Id: I35300149f61089d4325532c45329e0fb4df6c009
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/amboar/obmc-scripts/maintainers/obmc-gerrit b/amboar/obmc-scripts/maintainers/obmc-gerrit
index a52abbc..800e561 100755
--- a/amboar/obmc-scripts/maintainers/obmc-gerrit
+++ b/amboar/obmc-scripts/maintainers/obmc-gerrit
@@ -21,6 +21,8 @@
     if not root:
         root = git('rev-parse', '--show-toplevel').strip()
     mfile = os.path.join(root, mname)
+    if not os.path.exists(mfile):
+        return reviewers
     with open(mfile, 'r') as mstream:
         maintainers.trash_preamble(mstream)
         block = maintainers.parse_block(mstream)
@@ -43,6 +45,8 @@
 
 def decorate_refspec(refspec: str) -> str:
     gargs = gerrit_refspec_args(get_reviewers())
+    if not gargs:
+        return refspec
     if '%' in refspec:
         return "{},{}".format(refspec, gargs)
     return "{}%{}".format(refspec, gargs)