bug fix to 2218_team8

[windlikelyc]

What is the purpose of the change

#2218
When user use telnet to invoke org.apache.dubbo.demo.DemoService#sayHello and the parameter
is null, he gets a response called "null".The change is to make response more understandably.

Brief changelog

When catch NPE,return "parameters can't be null"

Verifying this change

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[codecov-io]

Codecov Report

Merging #2237 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master    #2237      +/-   ##
============================================
- Coverage     54.57%   54.55%   -0.03%     
+ Complexity     5156     5153       -3     
============================================
  Files           568      568              
  Lines         25019    25021       +2     
  Branches       4456     4457       +1     
============================================
- Hits          13655    13649       -6     
- Misses         9323     9327       +4     
- Partials       2041     2045       +4

Impacted Files	Coverage Δ	Complexity Δ
.../remoting/telnet/support/TelnetHandlerAdapter.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...ubbo/rpc/protocol/dubbo/ChannelWrappedInvoker.java	41.66% <0%> (-8.34%)	3% <0%> (ø)
...org/apache/dubbo/rpc/filter/ActiveLimitFilter.java	83.33% <0%> (-5.56%)	6% <0%> (-1%)
...rpc/protocol/dubbo/telnet/InvokeTelnetHandler.java	51.89% <0%> (-3.8%)	11% <0%> (-2%)
...onfig/spring/extension/SpringExtensionFactory.java	85.71% <0%> (+10.71%)	10% <0%> (ø)	⬇️

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