Painless: Disable java-9 indy string thing

It is breaking some of the doc tests. Also add a unit test that
shows the failure.

Relates to #18929

modules/lang-painless/src/main/java/org/elasticsearch/painless/WriterConstants.java

@@ -131,7 +131,7 @@ public final class WriterConstants {
             // not Java 9 - we set it null, so MethodWriter uses StringBuilder:
             bs = null;
         }
-        INDY_STRING_CONCAT_BOOTSTRAP_HANDLE = bs;
+        INDY_STRING_CONCAT_BOOTSTRAP_HANDLE = null; // Disabled until https://github.com/elastic/elasticsearch/issues/18929
     }
 
     public final static int MAX_INDY_STRING_CONCAT_ARGS = 200;

modules/lang-painless/src/test/java/org/elasticsearch/painless/StringTests.java

@@ -221,4 +221,8 @@ public class StringTests extends ScriptTestCase {
             exec("def x = null; def y = null; x += y");
         });
     }
+
+    public void testAppendStringIntoMap() {
+        assertEquals("nullcat", exec("def a = new HashMap(); a.cat += 'cat'"));
+    }
 }