return null when EmptyResultDataAccessException occurred for ldap scenarios

[nobodyiam]

What's the purpose of this PR

fix the issue that unexpected exception occurred when the searched user doesn't exist in the ldap system.

Which issue(s) this PR fixes:

Fixes #3394

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• Read the Contributing Guide before making this pull request.
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[codecov-io]

Codecov Report

Merging #3430 (20e6499) into master (f8683a2) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #3430      +/-   ##
============================================
- Coverage     51.40%   51.37%   -0.03%     
+ Complexity     2312     2311       -1     
============================================
  Files           441      441              
  Lines         13817    13821       +4     
  Branches       1407     1407              
============================================
- Hits           7102     7100       -2     
- Misses         6227     6232       +5     
- Partials        488      489       +1     

Impacted Files	Coverage Δ	Complexity Δ
...mework/apollo/portal/spi/ldap/LdapUserService.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...rk/apollo/spring/property/SpringValueRegistry.java	83.33% <0.00%> (-5.56%)	10.00% <0.00%> (-1.00%)
.../apollo/internals/RemoteConfigLongPollService.java	77.71% <0.00%> (-0.61%)	28.00% <0.00%> (-1.00%)
...ervice/service/ReleaseMessageServiceWithCache.java	85.88% <0.00%> (+1.17%)	24.00% <0.00%> (+1.00%)

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