main-eisop: Unbounded wildcards default to non-nullable

[cpovirk]

This is at least a contributor to perhaps 500ish [edit: probably even more!] of the 800ish errors that I've seen in testing inside Google so far:

import org.jspecify.annotations.NullMarked;
import org.jspecify.annotations.Nullable;

@NullMarked
class UnboundedDefaultsToNonNull<E extends @Nullable Object> {
  UnboundedDefaultsToNonNull<?> x() {
    return this;
  }
}

$ ../checker-framework/checker/bin/javac -processorpath $HOME/.m2/repository/org/jspecify/jspecify/0.3.0/jspecify-0.3.0.jar:build/libs/jspecify-reference-checker-0.0.0-SNAPSHOT.jar -processor com.google.jspecify.nullness.NullSpecChecker -AcheckImpl -AassumePure -AsuppressWarnings=contracts.conditional.postcondition.false.methodref,contracts.conditional.postcondition.false.override,contracts.conditional.postcondition.true.methodref,contracts.conditional.postcondition.true.override,purity.methodref,purity.overriding,type.anno.before.decl.anno,type.anno.before.modifier -cp $HOME/.m2/repository/org/jspecify/jspecify/0.3.0/jspecify-0.3.0.jar:build/libs/jspecify-reference-checker-0.0.0-SNAPSHOT.jar UnboundedDefaultsToNonNull.java
UnboundedDefaultsToNonNull.java:7: error: [return.type.incompatible] incompatible types in return.
    return this;
           ^
  type of expression: UnboundedDefaultsToNonNull<E>
  method return type: UnboundedDefaultsToNonNull<? extends Object>
1 error

In @NullMarked code, UnboundedDefaultsToNonNull<?> should be treated like UnboundedDefaultsToNonNull<? extends @Nullable Object>, rather than like UnboundedDefaultsToNonNull<? extends [@NonNull] Object>. This is jspecify/checker-framework#2.

[wmdietl]

Fixed by #165.