fix: NPE on named provider init/shutdown

[liran2000]

fix NPE on named provider initialization.

Problem

When initializing named provider first time, NPE is thrown.
This may cause old provider to not shutdown properly.

Solution

Add null check on isProviderRegistered.

Steps to reproduce

1. Add dependency at pom.xml to see logs on console:

<dependency>
      <groupId>org.apache.logging.log4j</groupId>
      <artifactId>log4j-slf4j2-impl</artifactId>
      <version>2.20.0</version>
      <scope>test</scope>
</dependency>

2. Run relevant individual unit test, e.g. dev.openfeature.sdk.EventsTest.ApiEvents.NamedProvider.Initialization.
3. Observe console log. Without this fix, the following error appears:

java.lang.NullPointerException: null
	at java.util.concurrent.ConcurrentHashMap.containsValue(ConcurrentHashMap.java:979) ~[?:?]
	at dev.openfeature.sdk.ProviderRepository.isProviderRegistered(ProviderRepository.java:145) ~[classes/:?]
	at dev.openfeature.sdk.ProviderRepository.shutDownOld(ProviderRepository.java:138) ~[classes/:?]
	at dev.openfeature.sdk.ProviderRepository.initializeProvider(ProviderRepository.java:130) ~[classes/:?]
	at dev.openfeature.sdk.ProviderRepository.lambda$prepareAndInitializeProvider$2(ProviderRepository.java:115) ~[classes/:?]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539) ~[?:?]
	at java.util.concurrent.FutureTask.run(FutureTask.java:264) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[?:?]
	at java.lang.Thread.run(Thread.java:833) ~[?:?]

[codecov]

Codecov Report

Merging #595 (682b609) into main (a2e4894) will increase coverage by 0.12%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##               main     #595      +/-   ##
============================================
+ Coverage     94.53%   94.65%   +0.12%     
- Complexity      357      359       +2     
============================================
  Files            32       32              
  Lines           841      842       +1     
  Branches         51       52       +1     
============================================
+ Hits            795      797       +2     
+ Misses           25       24       -1     
  Partials         21       21              

Flag	Coverage Δ
unittests	94.65% <66.66%> (+0.12%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
...n/java/dev/openfeature/sdk/ProviderRepository.java	96.96% <66.66%> (-1.50%)	⬇️

... and 1 file with indirect coverage changes

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[toddbaert]

Nice find, thanks!

If you feel like it, you could use Optional here, but that's preference.

I will merge this by EOD.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication