Fix the NPE bug when passing null args and paramIdx is negative

[sczyh30]

Describe what this PR does / why we need it

Fix the NPE bug when passing null args to SphU.entry(xxx, args) and paramIdx of parameter flow rule is negative (< 0).

Does this pull request fix one issue?

Fixes #643

Describe how you did it

Check null args in checkParams. Also rearrange some code in ParamFlowSlot.

Describe how to verify it

Run the test cases.

Special notes for reviews

NONE

[codecov-io]

Codecov Report

Merging #642 into master will increase coverage by 0.51%.
The diff coverage is 86.36%.

@@             Coverage Diff              @@
##             master     #642      +/-   ##
============================================
+ Coverage     40.06%   40.57%   +0.51%     
- Complexity     1293     1307      +14     
============================================
  Files           284      284              
  Lines          8916     8919       +3     
  Branches       1192     1192              
============================================
+ Hits           3572     3619      +47     
+ Misses         4887     4835      -52     
- Partials        457      465       +8

Impacted Files	Coverage Δ	Complexity Δ
...sentinel/slots/block/flow/param/ParamFlowSlot.java	81.25% <86.36%> (+2.56%)	22 <8> (+2)	⬆️
...a/csp/sentinel/slots/statistic/base/LeapArray.java	67.64% <0%> (-2.95%)	33% <0%> (-1%)
...ibaba/csp/sentinel/eagleeye/EagleEyeLogDaemon.java	30.3% <0%> (+6.06%)	6% <0%> (+1%)	⬆️
...a/csp/sentinel/slots/statistic/base/LongAdder.java	34.04% <0%> (+17.02%)	11% <0%> (+7%)	⬆️
...a/csp/sentinel/slots/statistic/base/Striped64.java	61.45% <0%> (+35.41%)	11% <0%> (+6%)	⬆️

---

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 3eb4a37...7dbe45a. Read the comment docs.

[CarpenterLee]

LGTM