@Nullable all the way: null-safety at field level

This commits extends nullability declarations to the field level, formalizing the interaction between methods and their underlying fields and therefore avoiding any nullability mismatch.

Issue: SPR-15720

spring-expression/src/test/java/org/springframework/expression/spel/SpelReproTests.java

@@ -1768,6 +1768,7 @@ public class SpelReproTests extends AbstractExpressionTests {
 				};
 			}
 		});
+
 		result = spel.getValue(context);
 		assertNotNull(result);
 		assertTrue(result.getClass().isArray());