Refine nullability of PropertyResolver.getProperty(key: String)

Closes gh-35104

Author: sdeleuze

spring-core/src/main/kotlin/org/springframework/core/env/PropertyResolverExtensions.kt

@@ -37,7 +37,7 @@ operator fun PropertyResolver.get(key: String) : String? = getProperty(key)
  * @author Sebastien Deleuze
  * @since 5.1
  */
-inline fun <reified T> PropertyResolver.getProperty(key: String) : T? =
+inline fun <reified T : Any> PropertyResolver.getProperty(key: String) : T? =
 		getProperty(key, T::class.java)
 
 /**