Fix HugeConfig save to file is empty

[Linary]

fix #50

[codecov]

Codecov Report

Merging #51 into master will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##             master      #51   +/-   ##
=========================================
  Coverage     81.80%   81.80%           
- Complexity      685      687    +2     
=========================================
  Files            57       57           
  Lines          2083     2089    +6     
  Branches        304      304           
=========================================
+ Hits           1704     1709    +5     
- Misses          250      251    +1     
  Partials        129      129           

Impacted Files	Coverage Δ	Complexity Δ
...ava/com/baidu/hugegraph/version/CommonVersion.java	50.00% <ø> (ø)	1.00 <0.00> (ø)
...in/java/com/baidu/hugegraph/config/HugeConfig.java	83.07% <66.66%> (-1.67%)	18.00 <1.00> (+1.00)	⬇️
.../main/java/com/baidu/hugegraph/perf/Stopwatch.java	87.75% <0.00%> (+2.04%)	21.00% <0.00%> (+1.00%)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b4fb1f7...199dd3d. Read the comment docs.