Add computeIfAbsent variants for non-nullable types to stores

[marcphilipp]

Since both ExtensionContext.Store and its backing
NamespaceAwareStore implementation allow storing null values, the
getOrComputeIfAbsent methods potentially returned null, even if
documented otherwise (in case of getOrComputeIfAbsent(Class) in
ExtensionContext.Store). Now that the API uses JSpecify's nullability
annotations, this became more apparent and also caused
Jupiter Extension implementations to have to deal with values being
potentially null despite their best intentions to only use
getOrComputeIfAbsent to access a certain key with a function that
never returned null. Therefore, this commit introduces null-safe
variants called computeIfAbsent that treat null as absent in
addition to the key not being present in the backing Map.

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes