Don't throw if no RPMServiceManager

[jeffalder]

Overview

We occasionally get an NPE in our logs

java.lang.NullPointerException: Cannot invoke "com.newrelic.agent.RPMServiceManager.getRPMService()" because the return value of "com.newrelic.agent.service.ServiceManager.getRPMServiceManager()" is null
	at com.newrelic.agent.service.ServiceFactory.getRPMService(ServiceFactory.java:103)
	at com.newrelic.agent.AgentImpl.getLinkingMetadata(AgentImpl.java:154)
	at com.newrelic.agent.bridge.logging.AppLoggingUtils.getLinkingMetadataBlob(AppLoggingUtils.java:57)

It looks like it comes from here, so I added a guard. AgentLinkingMetadata.getLinkingMetadata already handles the case if IRPMService is null, but I figured some linking metadata was better than none.

An alternative implementation would simply bypass this entirely if there was no entity guid yet. I'm not sure which way we want to go, but not getting an NPE seems preferable.

Related Github Issue

Fixes #1553

Testing

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Checks

• Your contributions are backwards compatible with relevant frameworks and APIs.
• Your code does not contain any breaking changes. Otherwise please describe.
• Your code does not introduce any new dependencies. Otherwise please describe.

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (68f1efb) 70.64% compared to head (0fbaa92) 70.64%.

Files	Patch %	Lines
...nt/src/main/java/com/newrelic/agent/AgentImpl.java	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1604   +/-   ##
=========================================
  Coverage     70.64%   70.64%           
  Complexity     9912     9912           
=========================================
  Files           826      826           
  Lines         39799    39802    +3     
  Branches       6009     6010    +1     
=========================================
+ Hits          28117    28120    +3     
+ Misses         8970     8969    -1     
- Partials       2712     2713    +1     

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[jasonjkeller]

@jeffalder Thanks for the fix. This looks good to me. We'll get it into the next agent release.