[BUG] Nullable annotation is not copied to builder

[lbenedetto]

Describe the bug
When a builder is generated for a class with a field marked as Nullable, the nullability is not transferred to the builder, causing warnings when trying to build an instance of that object using nullable variables.

To Reproduce
With lombok.addNullAnnotations = spring and a package-info.java with this:

@NonNullApi
package com.example;

import org.springframework.lang.NonNullApi;

Then when you make an object like this:

import org.springframework.lang.Nullable;
import lombok.Builder;

@Builder
public class Thing {
  @Nullable
  String thing;
}

it becomes

import org.springframework.lang.Nullable;

public class Thingy {
  @Nullable
  String thing;

  Thingy(@Nullable String thing) {
    this.thing = thing;
  }

  @org.springframework.lang.NonNull
  public static ThingBuilder builder() {
    return new ThingBuilder();
  }

  public static class ThingyBuilder {
    private String thing;

    ThingyBuilder() {
    }

    @org.springframework.lang.NonNull
    public ThingyBuilder thing(String thing) {
      this.thing = thing;
      return this;
    }

    @org.springframework.lang.NonNull
    public Thingy build() {
      return new Thingy(this.thing);
    }

    @org.springframework.lang.NonNull
    public String toString() {
      return "Thingy.ThingyBuilder(thing=" + this.thing + ")";
    }
  }
}

Expected behavior
Nullable annotation is copied to the builder like so:

import org.springframework.lang.Nullable;

public class Thingy {
  @Nullable
  String thing;

  Thingy(@Nullable String thing) {
    this.thing = thing;
  }

  @org.springframework.lang.NonNull
  public static ThingyBuilder builder() {
    return new ThingyBuilder();
  }

  public static class ThingyBuilder {
    @Nullable
    private String thing;

    ThingyBuilder() {
    }

    @org.springframework.lang.NonNull
    public ThingyBuilder thing(@Nullable String thing) {
      this.thing = thing;
      return this;
    }

    @org.springframework.lang.NonNull
    public Thingy build() {
      return new Thingy(this.thing);
    }

    @org.springframework.lang.NonNull
    public String toString() {
      return "Thingy.ThingyBuilder(thing=" + this.thing + ")";
    }
  }
}

Doing this would not interfere with the implementation of Builder.Default, since you generate a separate field for tracking if the field has already been manually set. With these changes I can do the following without any nullability warnings:

return Thingy.builder()
  .thing(mightBeNull)
  .build()

Currently, I have to do this instead:

var builder = Thingy.builder();
if (mightBeNull != null) {
  builder.thing(mightBeNull);
}
return builder.build();

Version info (please complete the following information):

• Lombok version: 1.18.36
• Delombok version (used for examining the generated code): 1.18.4.5
• Platform 21.0.3