@Override
    public Void visitTraversalSourceSpawnMethod_mergeE_Map(final GremlinParser.TraversalSourceSpawnMethod_mergeE_MapContext ctx){
        sb.append("mergeE(");
        tryCastMapNullableArgument(ctx.genericMapNullableArgument());
        sb.append(")");
        return null;
    }
Special handling for inject with second `null` argument like g.inject(1, null)
inject() ends up being ambiguous with groovy's jdk extension of inject(Object initialValue, Closure closure)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/language/translator/GroovyTranslateVisitor.java