Fix the Base64-decoder failing if JWT_SECRET is not provided

[burmanm]

If the JWT_SECRET is not specified, rest of the signing options are expected to be in Base64-format (which they're not). This modification requires only the JWT_SECRET to be in Base64-format, rest are not decoded anymore.

This should also fix #767 since the shortest Base64 string is 4 characters.

The Base64.getDecoder() is more strict and as such, this will test if those inputs are really Base64 or not (and current tests are enough to test this new behavior).

[codecov]

Codecov Report

Merging #1118 (676958e) into master (a13eb66) will decrease coverage by 0.29%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1118      +/-   ##
==========================================
- Coverage   74.27%   73.98%   -0.30%     
==========================================
  Files         133      133              
  Lines        9786     9788       +2     
  Branches     1018     1018              
==========================================
- Hits         7269     7242      -27     
- Misses       1926     1951      +25     
- Partials      591      595       +4     

Impacted Files	Coverage Δ
...ssandrareaper/resources/auth/ShiroJwtProvider.java	56.25% <66.66%> (-0.90%)	⬇️
...va/io/cassandrareaper/storage/PostgresStorage.java	70.72% <0.00%> (-4.57%)	⬇️
.../io/cassandrareaper/service/SchedulingManager.java	69.07% <0.00%> (-1.98%)	⬇️
...main/java/io/cassandrareaper/jmx/JmxProxyImpl.java	53.44% <0.00%> (-0.72%)	⬇️
...java/io/cassandrareaper/service/RepairManager.java	77.43% <0.00%> (-0.45%)	⬇️
...java/io/cassandrareaper/service/SegmentRunner.java	76.81% <0.00%> (-0.23%)	⬇️
...a/io/cassandrareaper/storage/CassandraStorage.java	83.34% <0.00%> (ø)
.../java/io/cassandrareaper/service/RepairRunner.java	83.73% <0.00%> (+1.20%)	⬆️

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[michaelsembwever]

LGTM. Thanks @burmanm