templater: fix crash by empty group expression

'exp' may be None because of '(group None)' node. The error message is copied
from revset.py.

mercurial/templater.py

@@ -259,6 +259,8 def prettyformat(tree):
 
 def compileexp(exp, context, curmethods):
     """Compile parsed template tree to (func, data) pair"""
+    if not exp:
+        raise error.ParseError(_("missing argument"))
     t = exp[0]
     if t in curmethods:
         return curmethods[t](exp, context)

tests/test-command-template.t

@@ -2766,6 +2766,16 Error on syntax:
   hg: parse error at 6: invalid token
   [255]
 
+  $ hg log -T '{}'
+  hg: parse error at 2: not a prefix: end
+  [255]
+  $ hg debugtemplate -v '{()}'
+  (template
+    (group
+      None))
+  hg: parse error: missing argument
+  [255]
+
 Behind the scenes, this will throw TypeError
 
   $ hg log -l 3 --template '{date|obfuscate}\n'