Fix the NPE occurred when using EnableApolloConfig with Spring 3.1.1

[nobodyiam]

What's the purpose of this PR

Fix the NPE occurred when using EnableApolloConfig with Spring 3.1.1 in master branch

Which issue(s) this PR fixes:

Fixes #4178

Brief changelog

• Disable namespace placeholder support for Spring version prior to 3.2.x and add necessary warn logs

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• Read the Contributing Guide before making this pull request.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit tests to verify the code.
• Run mvn clean test to make sure this pull request doesn't break anything.
• Update the CHANGES log.

[codecov-commenter]

Codecov Report

Merging #4180 (6a23d8b) into master (7c6c535) will increase coverage by 0.00%.
The diff coverage is 11.11%.

@@            Coverage Diff            @@
##             master    #4180   +/-   ##
=========================================
  Coverage     52.57%   52.57%           
- Complexity     2616     2620    +4     
=========================================
  Files           484      484           
  Lines         15192    15201    +9     
  Branches       1571     1572    +1     
=========================================
+ Hits           7987     7992    +5     
- Misses         6646     6651    +5     
+ Partials        559      558    -1     

Impacted Files	Coverage Δ
...spring/spi/DefaultApolloConfigRegistrarHelper.java	74.19% <11.11%> (-25.81%)	⬇️
...ervice/service/ReleaseMessageServiceWithCache.java	87.05% <0.00%> (+1.17%)	⬆️
.../apollo/internals/RemoteConfigLongPollService.java	78.31% <0.00%> (+1.20%)	⬆️
...nfigservice/filter/ClientAuthenticationFilter.java	93.61% <0.00%> (+2.12%)	⬆️

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