return ajacent vertex with undefined label if custom vertex id override

[javeme]

Change-Id: Ia1940db01ea135d40621bb2579ed82bdba381ebd
Defined-Behavior: #1179

[codecov]

Codecov Report

Merging #1192 into master will increase coverage by 35.92%.
The diff coverage is 85.27%.

@@              Coverage Diff              @@
##             master    #1192       +/-   ##
=============================================
+ Coverage     29.65%   65.58%   +35.92%     
- Complexity      263     5728     +5465     
=============================================
  Files            75      361      +286     
  Lines          4208    29546    +25338     
  Branches        510     4172     +3662     
=============================================
+ Hits           1248    19377    +18129     
- Misses         2797     8190     +5393     
- Partials        163     1979     +1816     

Impacted Files	Coverage Δ	Complexity Δ
...a/com/baidu/hugegraph/variables/HugeVariables.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
.../hugegraph/backend/serializer/TableSerializer.java	86.72% <66.66%> (ø)	57.00 <0.00> (?)
...ava/com/baidu/hugegraph/structure/HugeElement.java	69.66% <71.42%> (ø)	75.00 <7.00> (?)
...java/com/baidu/hugegraph/structure/HugeVertex.java	78.65% <81.63%> (ø)	87.00 <17.00> (?)
...om/baidu/hugegraph/backend/store/ram/RamTable.java	66.89% <90.00%> (ø)	33.00 <4.00> (?)
...n/java/com/baidu/hugegraph/structure/HugeEdge.java	76.49% <93.10%> (ø)	86.00 <8.00> (?)
.../com/baidu/hugegraph/auth/RelationshipManager.java	81.63% <100.00%> (ø)	30.00 <0.00> (?)
...in/java/com/baidu/hugegraph/backend/id/EdgeId.java	77.67% <100.00%> (ø)	44.00 <2.00> (?)
...hugegraph/backend/serializer/BinarySerializer.java	85.00% <100.00%> (ø)	116.00 <0.00> (?)
...u/hugegraph/backend/serializer/TextSerializer.java	86.53% <100.00%> (ø)	96.00 <0.00> (?)
... and 289 more

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[houzhizhen]

ensureIsNumber is better