fix(core): allow zero max size caches across providers

[iProdigy]

Some users may do maxSize(0L) or expiryTime(Duration.ZERO) to "disable" caching.

However, this is not supported by all of the underlying providers: https://togithub.com/FasterXML/jackson-databind/pull/4115#discussion_r1335062530

This PR returns EmptyCache.INSTANCE in these scenarios to avoid exceptions/inconsistency across providers.

[iProdigy]

Open question: Users may assume compute, computeIfAbsent, merge always yield not-null values - should we evaluate (and return) the value from the passed lambdas instead of null (despite never writing the value)?

Edit: actually this behavior could be interpreted as performing a write & immediately evicting it, so I will proceed with the patch

[iProdigy]

for example:

[sonarqubecloud]

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