ui: fix echo back of ui.prompt() to not concatenate None as bytes stable

ui: fix echo back of ui.prompt() to not concatenate None as bytes

Spotted while writing tests for the issue6425. The default value may be
None.

--- a/mercurial/ui.py	Thu Oct 29 07:51:13 2020 +0100
+++ b/mercurial/ui.py	Fri Oct 23 20:10:17 2020 +0900
@@ -1659,7 +1659,9 @@
             if not r:
                 r = default
             if self.configbool(b'ui', b'promptecho'):
-                self._writemsg(self._fmsgout, r, b"\n", type=b'promptecho')
+                self._writemsg(
+                    self._fmsgout, r or b'', b"\n", type=b'promptecho'
+                )
             return r
         except EOFError:
             raise error.ResponseExpected()