Use Assert.notNull for null check.

Closes #3441

Author: dukbong

spring-data-jpa/src/main/java/org/springframework/data/jpa/mapping/JpaMetamodelMappingContext.java

@@ -136,10 +136,8 @@ public JpaMetamodel getRequiredMetamodel(TypeInformation<?> type) {
 
 			JpaMetamodel metamodel = getMetamodel(type);
 
-			if (metamodel == null) {
-				throw new IllegalArgumentException(String.format("Required JpaMetamodel not found for %s", type));
-			}
-
+			Assert.notNull(metamodel, () -> String.format("Required JpaMetamodel not found for %s", type));
+			
 			return metamodel;
 		}