Require explicit constraints in @⁠Contract

Prior to this commit, the value attribute in @⁠Contract was declared as
follows.

    String value() default "";

That allowed empty declarations such as @⁠Contract or @⁠Contract();
however, a contract is not useful without declared constraints.

To address that, this commit removes the `default ""` declaration in
order to force users to provide explicit constraints.

Although one could technically still declare the annotation without
constraints via @⁠Contract(""), it's unlikely that anyone would
intentionally do that.

Closes gh-35157

Author: sbrannen

spring-core/src/main/java/org/springframework/lang/Contract.java

@@ -81,6 +81,6 @@
 	/**
 	 * Contains the contract clauses describing causal relations between call arguments and the returned value.
 	 */
-	String value() default "";
+	String value();
 
 }