bugfix: fix RpcContext.addResource when resource is null

[dk-lockdown]

RpcContext.addResource(String resource),when resource is blank,should do nothing,instead of reset resourceSets

Ⅰ. Describe what this PR did

the original code is

public void addResource(String resource) {
        if (null == resource) {
            this.resourceSets = new HashSet<String>();
        }
        this.resourceSets.add(resource);
    }

obviously, there is a bug

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #2490 into develop will decrease coverage by 0.03%.
The diff coverage is 60.00%.

@@              Coverage Diff              @@
##             develop    #2490      +/-   ##
=============================================
- Coverage      51.39%   51.36%   -0.04%     
  Complexity      2664     2664              
=============================================
  Files            529      529              
  Lines          16981    16983       +2     
  Branches        2050     2051       +1     
=============================================
- Hits            8728     8723       -5     
- Misses          7426     7432       +6     
- Partials         827      828       +1     

Impacted Files	Coverage Δ	Complexity Δ
...re/src/main/java/io/seata/core/rpc/RpcContext.java	40.54% <60.00%> (-3.91%)	21.00 <4.00> (ø)
...erver/storage/file/session/FileSessionManager.java	57.63% <0.00%> (-2.09%)	31.00% <0.00%> (-1.00%)

[jsbxyyx]

please git relation github user.name and user.email

[slievrly]

I made sure that this method is only used in UT.

[slievrly]

LGTM

[xingfudeshi]

LGTM.