[BUG] Results Parsing Error When Label is None in Gremlin Queries

**Describe the bug**

When executing certain Gremlin queries that involve elementMap(), the labels that can be None if the element is None. For example:
```
g.V(1).union(outE().as('e'), out().as('middle').outE().as('e')).dedup().otherV().as('endv').select('endv', 'e', 'middle').by(elementMap()).by(elementMap()).by(elementMap()).limit(1)
```
In this query, the `middle` vertex can be None, and the elementMap() results for the `middle` vertex looks like (~id, ~label, and its properties are all None):
```
KeyValue { key: Some(Value { item: Some(Str("middle")) }), value: Some(Entry { inner: Some(Map(KeyValues { key_values: [KeyValue { key: Some(Value { item: Some(Str("~label")) }), value: Some(Entry { inner: Some(Element(Element { inner: Some(Object(Value { item: Some(None(None)) })) })) }) }, KeyValue { key: Some(Value { item: Some(Str("~id")) }), value: Some(Entry { inner: Some(Element(Element { inner: Some(Object(Value { item: Some(None(None)) })) })) }) }, KeyValue { key: Some(Value { item: Some(Str("user_email")) }), value: Some(Entry { inner: Some(Element(Element { inner: Some(Object(Value { item: Some(None(None)) })) })) }) }, KeyValue { key: Some(Value { item: Some(Str("entity_value")) }), value: Some(Entry { inner: Some(Element(Element { inner: Some(Object(Value { item: Some(None(None)) })) })) }) }, KeyValue { key: Some(Value { item: Some(Str("user_nick")) }), value: Some(Entry { inner: Some(Element(Element { inner: Some(Object(Value { item: Some(None(None)) })) })) }) }] })) }) }] })) }) }] }
```

However, in result parsing, the compiler returns error message:
```
ErrorCode: GREMLIN_INVALID_RESULT\nMessage: cannot parse label value with type=NONE\nEC: 03-0108\nstacktrace: java.lang.IllegalArgumentException: cannot parse label value with type=NONE\n\tat com.alibaba.graphscope.common.result.Utils.parseLabelValue(Utils.java:145)\n\tat com.alibaba.graphscope.gremlin.resultx.GremlinRecordParser.parseValue(GremlinRecordParser.java:267)\n\tat com.alibaba.graphscope.gremlin.resultx.GremlinRecordParser.parseElement(GremlinRecordParser.java:214)\n\tat com.alibaba.graphscope.gremlin.resultx.GremlinRecordParser.parseEntry(GremlinRecordParser.java:130)\n\tat com.alibaba.graphscope.gremlin.resultx.GremlinRecordParser.parseKeyValues(GremlinRecordParser.java:180)\n\tat com.alibaba.graphscope.gremlin.resultx.GremlinRecordParser.parseEntry(GremlinRecordParser.java:124)\n\tat com.alibaba.graphscope.gremlin.resultx.GremlinRecordParser.parseKeyValues(GremlinRecordParser.java:180)\n\tat com.alibaba.graphscope.gremlin.resultx.GremlinRecordParser.parseEntry(GremlinRecordParser.java:124)\n\tat com.alibaba.graphscope.gremlin.resultx.GremlinRecordParser.parseFrom(GremlinRecordParser.java:95)\n\tat com.alibaba.graphscope.gremlin.resultx.GremlinResultProcessor.lambda$processRecord$0(GremlinResultProcessor.java:134)\n\tat com.alibaba.graphscope.common.utils.ClassUtils.callExceptionWithDetails(ClassUtils.java:54)\n\tat com.alibaba.graphscope.common.utils.ClassUtils.callException(ClassUtils.java:48)\n\tat com.alibaba.graphscope.gremlin.resultx.GremlinResultProcessor.processRecord(GremlinResultProcessor.java:133)\n\tat com.alibaba.graphscope.gremlin.resultx.GremlinResultProcessor.request(GremlinResultProcessor.java:92)\n\tat com.alibaba.graphscope.gremlin.plugin.processor.LifeCycleSupplier.lambda$get$2(LifeCycleSupplier.java:136)\n\tat org.apache.tinkerpop.gremlin.groovy.engine.GremlinExecutor.lambda$eval$0(GremlinExecutor.java:283)\n\tat java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)\n\tat java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)\n\tat java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)\n\tat java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)\n\tat java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)\n\tat java.base/java.lang.Thread.run(Thread.java:829)\n\nQueryId: 4950820082035874898\n
```

Moreover, if we use `valueMap()` in the query instead of `elementMap()`, i.e.,
```
g.V(1).union(outE().as('e'), out().as('middle').outE().as('e')).dedup().otherV().as('endv').select('endv', 'e', 'middle').by(valueMap()).by(valueMap()).by(valueMap()).limit(1)
```
the query successfully returns results as expected (with `valueMap` of `middle` as a None value). The expectation is that the results of elementMap() and valueMap() should be consistent.