[ruby/error_highlight] Fix the spurious TypeError.

When we have no backtrace locations, we can't have the highlight,
so just return the message.

https://github.com/ruby/error_highlight/commit/9f5c639494

lib/error_highlight/core_ext.rb

@@ -15,6 +15,7 @@ module ErrorHighlight
       return msg unless locs
 
       loc = locs.first
+      return msg unless loc
       begin
         node = RubyVM::AbstractSyntaxTree.of(loc, keep_script_lines: true)
         opts = {}

test/error_highlight/test_error_highlight.rb

@@ -1198,9 +1198,6 @@ undefined method `time' for 1:Integer
   def test_simulate_funcallv_from_embedded_ruby
     assert_error_message(NoMethodError, <<~END) do
 undefined method `foo' for nil:NilClass
-
-      nil.foo + 1
-         ^^^^
     END
 
       nil.foo + 1