Fix null map key parsing in GenericLiteralVisitor

The visitor treated all keyword map keys as their text representation,
so a null key in [null:"value"] was parsed as the String "null" instead
of Java null. This broke round-tripping maps with null keys through
GremlinLang serialization and ANTLR parsing.

Author: kenhuuu

gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/language/grammar/GenericLiteralVisitor.java

@@ -350,7 +350,8 @@ public Object visitGenericMapLiteral(final GremlinParser.GenericMapLiteralContex
             } else if (kctx instanceof GremlinParser.GenericMapLiteralContext) {
                 key = visitGenericMapLiteral((GremlinParser.GenericMapLiteralContext) kctx);
             } else if (kctx instanceof GremlinParser.KeywordContext) {
-                key = ((GremlinParser.KeywordContext) kctx).getText();
+                final String keywordText = ((GremlinParser.KeywordContext) kctx).getText();
+                key = keywordText.equals("null") ? null : keywordText;
             } else if (kctx instanceof GremlinParser.NakedKeyContext) {
                 key = ((GremlinParser.NakedKeyContext) kctx).getText();
             } else if (kctx instanceof TerminalNode) {

gremlin-core/src/test/java/org/apache/tinkerpop/gremlin/language/grammar/GeneralLiteralVisitorTest.java

@@ -632,6 +632,10 @@ public static Iterable<Object[]> generateTestParameters() {
                     {"[{'x'}: 'x']", createMap(new Object[]{new HashSet<>(Arrays.asList("x")), "x"})},
                     {"[['x']: ['x',{'y'}]]", createMap(new Object[]{Arrays.asList("x"), Arrays.asList("x", new HashSet<>(Arrays.asList("y")))})},
                     {"[['x']: ['x',['y']]]", createMap(new Object[]{Arrays.asList("x"), Arrays.asList("x", Arrays.asList("y"))})},
+                    {"[null:'x']", createMap(new Object[]{null, "x"})},
+                    {"[null:null]", createMap(new Object[]{null, null})},
+                    {"[null:'x','y':'z']", createMap(new Object[]{null, "x", "y", "z"})},
+                    {"['null':'x']", createMap(new Object[]{"null", "x"})},
             });
         }