[ruby/prism] Amend assert in pm_utf_8_codepoint(), n=0 is fine

This assert used to trip in the included test:
```
./miniruby --parser=prism -e ' "%W"\u" '
```

https://github.com/ruby/prism/commit/8c0f84db4f

1 files changed, 7 insertions, 0 deletions

diff --git a/test/prism/format_errors_test.rb b/test/prism/format_errors_test.rb
index 34d320a7b6..bc0b26165d 100644
--- a/test/prism/format_errors_test.rb
+++ b/test/prism/format_errors_test.rb
@@ -12,6 +12,13 @@ module Prism
             | ^ unexpected '<', ignoring it
             |  ^ unexpected '>', ignoring it
       ERROR
+
+      assert_equal <<~'ERROR', Debug.format_errors('"%W"\u"', false)
+        > 1 | "%W"\u"
+            |     ^ expected a newline or semicolon after the statement
+            |     ^ invalid token
+            |        ^ expected a closing delimiter for the string literal
+      ERROR
     end
   end
 end