Permit assignment to @MonotonicNonNull in constructor

Every possibly-nullable assignment to a @MonotonicNonNull field is prohibited.
However, such an assignment should be permitted as the first assignment to the field in a constructor, if the field was not initialized to a possibly non-null value at its declaration. (That is, it was not initialized, or it was initialized to null.)
Here is an example:

import org.checkerframework.checker.nullness.qual.Nullable;
import org.checkerframework.checker.nullness.qual.MonotonicNonNull;

class Issue2215Parent {
    @MonotonicNonNull Object a;
    @MonotonicNonNull Object b = null;
    @MonotonicNonNull Object c = possiblyNull();
    @MonotonicNonNull Object d = new Object();

    static @Nullable Object possiblyNull() {
        return null;
    }
}

public class Issue2215 extends Issue2215Parent {

  @MonotonicNonNull Object f;
  @MonotonicNonNull Object g = null;
  @MonotonicNonNull Object h = Issue2215Parent.possiblyNull();
  @MonotonicNonNull Object i = new Object();

  Issue2215(@Nullable Object o) {

    this.a = o;
    this.b = o;
    // :: error: (assignment.type.incompatible)
    this.c = o;
    // :: error: (assignment.type.incompatible)
    this.d = o;
    // :: error: (assignment.type.incompatible)
    this.a = o;
    // :: error: (assignment.type.incompatible)
    this.b = o;
    // :: error: (assignment.type.incompatible)
    this.c = o;
    // :: error: (assignment.type.incompatible)
    this.d = o;

    this.f = o;
    this.g = o;
    // :: error: (assignment.type.incompatible)
    this.h = o;
    // :: error: (assignment.type.incompatible)
    this.i = o;
    // :: error: (assignment.type.incompatible)
    this.f = o;
    // :: error: (assignment.type.incompatible)
    this.g = o;
    // :: error: (assignment.type.incompatible)
    this.h = o;
    // :: error: (assignment.type.incompatible)
    this.i = o;

  }

}

Use the Java reproducer in the issue as the starting point and inspect the nullness-checker handling of @MonotonicNonNull field assignments during constructors. Verify that the first possibly-null assignment is accepted only for fields uninitialized or initialized to null, while later assignments and fields initialized to possibly or definitely non-null values still produce the shown diagnostics.