bugfix: check null channel

[lightClouds917]

Ⅰ. Describe what this PR did
if (null == rmChannel || rmChannel.isActive()) { return rmChannel; }
Our goal is to get a active rmChannel,So when null == rmChannel ,we can not return,Otherwise it will be called in a loop.

Ⅱ. Does this pull request fix one issue?

I dont know.

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

Ⅳ. Describe how to verify it

review maybe

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #1395 into develop will not change coverage.
The diff coverage is 0%.

@@            Coverage Diff             @@
##             develop    #1395   +/-   ##
==========================================
  Coverage      45.97%   45.97%           
  Complexity      1695     1695           
==========================================
  Files            345      345           
  Lines          12486    12486           
  Branches        1597     1597           
==========================================
  Hits            5741     5741           
  Misses          6092     6092           
  Partials         653      653

Impacted Files	Coverage Δ	Complexity Δ
...eata/core/rpc/netty/NettyClientChannelManager.java	70.53% <0%> (ø)	22 <0> (ø)	⬇️

---

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 5b9a0fa...d617feb. Read the comment docs.

[slievrly]

LGTM