Handle strftime("") correctly in the fast path

* Tested in test/mri/tests/ruby/test_time.rb

Author: eregon

src/main/java/org/truffleruby/core/time/RubyDateFormatter.java

@@ -692,6 +692,10 @@ public static Rope formatToRopeFast(Token[] compiledPattern, ZonedDateTime dt,
             }
         }
 
+        if (rope == null) {
+            rope = RopeConstants.EMPTY_UTF8_ROPE;
+        }
+
         return rope;
     }