bugfix:fix npe of AbstractCluster

[leizhiyuan]

Motivation:

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To make others understand what is the problem you're trying to solve.

Modification:

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Result:

Fixes ##724

If there is no issue then describe the changes introduced by this PR.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@201de55). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #795   +/-   ##
=========================================
  Coverage          ?   71.83%           
  Complexity        ?     1078           
=========================================
  Files             ?      376           
  Lines             ?    15344           
  Branches          ?     2442           
=========================================
  Hits              ?    11023           
  Misses            ?     3054           
  Partials          ?     1267

Impacted Files	Coverage Δ	Complexity Δ
...va/com/alipay/sofa/rpc/client/AbstractCluster.java	66.77% <100%> (ø)	0 <0> (?)

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[khotyn]

@leizhiyuan You should ask someone else to review your code.