erlang: Fix highlighting of empty records

For example

    -record(state, {})

Would not highlight `state` as a type since the alternation didn't
allow for an empty tuple. Allowing the inner atom of the tuple to be
optional fixes this case.

runtime/queries/erlang/highlights.scm

@@ -18,7 +18,7 @@
     .
     [(atom) @type (macro)]
     [
-      (tuple (atom) @variable.other.member)
+      (tuple (atom)? @variable.other.member)
       (tuple
         (binary_operator
           left: (atom) @variable.other.member