Spring Refine nullability of PropertyResolver.getProperty(key: String) extension

inline fun <reified T> PropertyResolver.getProperty(key: String) : T? should be refined to inline fun <reified T : Any> PropertyResolver.getProperty(key: String) : T? as the return value is always nullable, so the nullability of the generic type is not useful in that use case. That change will also improve the consistency against other PropertyResolverExtensions.kt extensions.

As a side effect, it workarounds https://youtrack.jetbrains.com/issue/KT-78555.