[Backport] Coerce blank fields to null in ApiKey requests

[tvernum]

API Key request objects (InvalidateApiKeyRequest and
GetApiKeyRequest) support multiple key-selection parameters such as
realm-name, user-name, key-id and key-name.

The specified behaviour is that if any of these are blank then they
act as a wildcard and do not restrict the search criteria.

This change moves the "is blank" logic into the constructor for these
requests so that there is a single consistent way to determine blank
(Strings.hasText(arg) == false) and all usage of these fields can
rely on the getter returning either null or a real value, and
never a non-null blank.

Backport of: #66240

[tvernum]

@elasticmachine run elasticsearch-ci/2

java.lang.AssertionError: transform [continuous-histogram-pivot-test] does not progress, state: INDEXING, reason: null	
Expected: a value greater than <1608007624392L>	
     but: <1608007601541L> was less than <1608007624392L>	
	at __randomizedtesting.SeedInfo.seed([2A594CAE4A2FA811:116F3DB5C0442A47]:0)	
		•••
	at org.elasticsearch.xpack.transform.integration.continuous.TransformContinuousIT.lambda$waitUntilTransformsReachedUpperBound$3(TransformContinuousIT.java:499)	
		•••
	at org.elasticsearch.xpack.transform.integration.continuous.TransformContinuousIT.waitUntilTransformsReachedUpperBound(TransformContinuousIT.java:497)	
	at org.elasticsearch.xpack.transform.integration.continuous.TransformContinuousIT.testContinousEvents(TransformContinuousIT.java:275)