ui: fix crash by non-interactive prompt echo for user name

Since we've dropped a str cast at write() by f04bd381e8c0, ui.prompt() should
convert default to '' if it is None. Otherwise, write() would fail with
"TypeError: object of type 'NoneType' has no len()".

This patch includes the tests for both interactive and non-interactive cases
because "ui.askusername" was never tested.

mercurial/ui.py

@@ -798,7 +798,7 b' class ui(object):'
         If ui is not interactive, the default is returned.
         """
         if not self.interactive():
-            self.write(msg, ' ', default, "\n")
+            self.write(msg, ' ', default or '', "\n")
             return default
         try:
             r = self._readline(self.label(msg, 'ui.prompt'))

tests/test-committer.t

@@ -66,6 +66,31 b''
   date:        Thu Jan 01 00:00:00 1970 +0000
   summary:     commit-1
   
+# test prompt username
+
+  $ cat > .hg/hgrc <<EOF
+  > [ui]
+  > askusername = True
+  > EOF
+
+  $ echo 12345 > asdf
+  $ hg commit --config ui.interactive=False -m ask
+  enter a commit username: 
+  no username found, using '[^']*' instead (re)
+  $ hg rollback -q
+
+  $ hg commit --config ui.interactive=True -m ask <<EOF
+  > Asked User <[e-mail]>
+  > EOF
+  enter a commit username: Asked User <[e-mail]>
+  $ hg tip
+  changeset:   5:84c91d963b70
+  tag:         tip
+  user:        Asked User <[e-mail]>
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     ask
+  
+
 # test no .hg/hgrc (uses generated non-interactive username)
 
   $ echo space > asdf