Fix overridden methods nullability [SPR-15869]

[spring-issuemaster]

Sébastien Deleuze opened SPR-15869 and commented

As specified in this custom nullability proposal and as discussed with Jetbrains team, methods overridding parent one with @Nullable return values annotation require the @Nullable annotation to be present at the same level than @Override to get nullable qualifier taken in account in the overridden method.

This is currently not done in most places, so this need to be fixed to get null-safety handled correctly by IDEA and Kotlin.

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Issue Links:

• #20099 Introduce null-safety of Spring Framework API
• #20311 Revisit nullability annotations towards GA

Referenced from: commits 73cf07e, 47a7475