templater: handle a missing value correctly

Before, using a broken style such as:
changeset =
would result in a traceback.

This fixes a regression introduced in 47618355ffc8.

mercurial/templater.py

@@ -295,6 +295,8 @@ class templater(object):
         conf.read(mapfile)
 
         for key, val in conf[''].items():
+            if not val:
+                raise SyntaxError(_('%s: missing value') % conf.source('', key))
             if val[0] in "'\"":
                 try:
                     self.cache[key] = parsestring(val)

tests/test-command-template.t

@@ -463,6 +463,13 @@ Error if style missing key:
   abort: "changeset" not in template map
   [255]
 
+Error if style missing value:
+
+  $ echo 'changeset =' > t
+  $ hg log --style t
+  abort: t:1: missing value
+  [255]
+
 Error if include fails:
 
   $ echo 'changeset = q' >> t