Allow case expressions to be empty

This is not technically valid but it improves highlighting for the
'end' token when writing out case expressions.

grammar.js

@@ -386,7 +386,7 @@ module.exports = grammar({
         "case",
         field("subject", $._expression),
         "of",
-        sep($.clause, ";"),
+        optional(sep($.clause, ";")),
         "end"
       ),
 

test/corpus/expressions.txt

@@ -736,3 +736,19 @@ Multi-line anonymous function within larger expression
               (arguments
                 (atom))))))
       (arguments))))
+
+================================================================================
+case with no body
+================================================================================
+
+%% This is not actually valid but improves highlighting.
+case Foo of
+end.
+
+--------------------------------------------------------------------------------
+
+(source
+  (line_comment
+    (comment_content))
+  (case
+    (variable)))

test/highlight/expressions.erl

@@ -39,3 +39,10 @@ State#state.config#config.pid
 %%                ^ operator
 %%                   ^ type
 %%                         ^ property
+
+case Foo of
+%% <- keyword
+%%    ^ variable
+%%       ^ keyword
+end
+%% <- keyword