Fix: attachments maybe nil or empty string

[PhilYue]

What this PR does:

Unmarshal request body by hession.go , the attribute attachments maybe nil or empty string, and will stop the invoked

Which issue(s) this PR fixes:

if attachments nil or empty string , default a value that for normal invoking

Fixes #1583

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[PhilYue]

#1583

[codecov-commenter]

Codecov Report

Merging #1631 (0d5ed1f) into 3.0 (f7f1581) will decrease coverage by 0.08%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##              3.0    #1631      +/-   ##
==========================================
- Coverage   41.10%   41.02%   -0.09%     
==========================================
  Files         253      253              
  Lines       14498    14498              
==========================================
- Hits         5960     5948      -12     
- Misses       7848     7861      +13     
+ Partials      690      689       -1     

Impacted Files	Coverage Δ
protocol/dubbo/impl/hessian.go	31.23% <0.00%> (ø)
metadata/report/delegate/delegate_report.go	26.49% <0.00%> (-7.95%)	⬇️

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