[java-analysis] Minor fix+javadoc for type nullability support (IDEA-372347)

GitOrigin-RevId: c35b63418dc3973e0b0f27f63f79fe465ae1b676

java/java-psi-api/src/com/intellij/codeInsight/TypeNullability.java

@@ -65,6 +65,11 @@ public final class TypeNullability {
     return inherited.equals(mySource) ? this : new TypeNullability(myNullability, inherited);
   }
 
+  /**
+   * @param nullability instantiation nullability
+   * @return the nullability of the instantiated type parameter,
+   * assuming that this object is the declared nullability of the type parameter.
+   */
   public @NotNull TypeNullability instantiatedWith(@NotNull TypeNullability nullability) {
     if (this.nullability() == nullability.nullability()) {
       return intersect(Arrays.asList(this, nullability));

java/java-psi-api/src/com/intellij/util/JavaTypeNullabilityUtil.java

@@ -82,7 +82,7 @@ public final class JavaTypeNullabilityUtil {
     for (PsiAnnotation annotation : annotations) {
       String qualifiedName = annotation.getQualifiedName();
       NullableNotNullManager manager = NullableNotNullManager.getInstance(annotation.getProject());
-      if (manager == null) continue;
+      if (manager == null) return TypeNullability.UNKNOWN;
       Optional<Nullability> optionalNullability = manager.getAnnotationNullability(qualifiedName);
       if (optionalNullability.isPresent()) {
         Nullability nullability = optionalNullability.get();