Fix returning the 500 error response when invoking the API after expiring the generated refresh token for the endpoint security

[YasasRangika]

Purpose

To fix the 500 error response when invoking the API after expiring the generated refresh token for the endpoint security with client credentials grant.

Goal

Fixes: wso2/api-manager#2261

Approach

Change the order of checking the refresh token availability to prevent this behavior with client credentials grant.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (1cfcab5) 39.07% compared to head (84f1849) 47.98%.
Report is 9 commits behind head on master.

Files	Patch %	Lines
...gt/gateway/mediators/oauth/client/OAuthClient.java	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #12185      +/-   ##
============================================
+ Coverage     39.07%   47.98%   +8.90%     
- Complexity        0     4024    +4024     
============================================
  Files          1785     1785              
  Lines        130443   130470      +27     
  Branches      18795    18801       +6     
============================================
+ Hits          50967    62601   +11634     
+ Misses        73003    60624   -12379     
- Partials       6473     7245     +772     

Flag	Coverage Δ
integration_tests	42.81% <75.00%> (+11.45%)	⬆️
unit_tests	20.84% <0.00%> (+<0.01%)	⬆️

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