Fix NPE in CrudRepositoryExtensions

[OsaSoft]

Erasure of nullability information was causing NPEs in the forced non-null assertion, as it allowed to pass a nullable value, see following snippet that would pass compilation correctly, but throw an unexpected NPE:

interface Repo : ListCrudRepository<User, String>

	val repo: Repo = mockk()

	@Test
	fun `CrudRepository#findByIdOrNull() extension should not throw NPE`() {
		val id: String? = null
		// compiler allows null value to be passed to findByIdOrNull, so NPE is thrown
		// I believe this is we lose the non-null type info when the repo extends a java interface,
		// which extends CrudRepository
		repo.findByIdOrNull(id)
	}

The fix enforces the generic ID type to be a not-null type, therefore the above code no longer compiles.

Not sure how to add a test for this, since the fix is on a static-type level and the problematic code no longer compiles. Could add in a comment, I guess?

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[mp911de]

The type should be non-nullable indeed. We would incorporate this change for the next GA release only without backporting it as this is a breaking change in some sense regarding compiling.