Prevent repo info from crashing when default element doesn't exist.

repo info will crash when using a manifest with no default element despite
default being an optional element. Output nothing for "Manifest Branch" if no
default element exists (or if no default revision exists).

Change-Id: I7ebffa2408863837ba980f0ab6e593134400aea9

subcmds/info.py

@@ -59,7 +59,8 @@ class Info(PagedCommand):
                       or 'all,-notdefault')
 
     self.heading("Manifest branch: ")
-    self.headtext(self.manifest.default.revisionExpr)
+    if self.manifest.default.revisionExpr:
+        self.headtext(self.manifest.default.revisionExpr)
     self.out.nl()
     self.heading("Manifest merge branch: ")
     self.headtext(mergeBranch)