Fix npe because missing edge mapping struct

[Linary]

Change-Id: I79e0c2a2d3e232cd2a5adb945214cbe3cbfe3b43

[codecov]

Codecov Report

Merging #213 (ad81ca5) into master (05967f0) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #213      +/-   ##
============================================
+ Coverage     74.04%   74.07%   +0.02%     
- Complexity      871      872       +1     
============================================
  Files            80       80              
  Lines          3641     3645       +4     
  Branches        436      436              
============================================
+ Hits           2696     2700       +4     
  Misses          745      745              
  Partials        200      200              

Impacted Files	Coverage Δ	Complexity Δ
...va/com/baidu/hugegraph/loader/HugeGraphLoader.java	92.42% <100.00%> (+0.03%)	45.00 <0.00> (ø)
...om/baidu/hugegraph/loader/metrics/LoadSummary.java	92.18% <100.00%> (+0.38%)	28.00 <3.00> (+1.00)

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