[fixes #1812] @Singular marked collections which nevertheless somehow ended up null would cause an NPE during toBuilder() invocations.

test/transform/resource/after-ecj/BuilderSingularToBuilderWithNull.java

@@ -0,0 +1,57 @@
+import lombok.Singular;
+@lombok.Builder(toBuilder = true) class BuilderSingularToBuilderWithNull {
+  public static @java.lang.SuppressWarnings("all") class BuilderSingularToBuilderWithNullBuilder {
+    private @java.lang.SuppressWarnings("all") java.util.ArrayList<String> elems;
+    @java.lang.SuppressWarnings("all") BuilderSingularToBuilderWithNullBuilder() {
+      super();
+    }
+    public @java.lang.SuppressWarnings("all") BuilderSingularToBuilderWithNullBuilder elem(String elem) {
+      if ((this.elems == null))
+          this.elems = new java.util.ArrayList<String>();
+      this.elems.add(elem);
+      return this;
+    }
+    public @java.lang.SuppressWarnings("all") BuilderSingularToBuilderWithNullBuilder elems(java.util.Collection<? extends String> elems) {
+      if ((this.elems == null))
+          this.elems = new java.util.ArrayList<String>();
+      this.elems.addAll(elems);
+      return this;
+    }
+    public @java.lang.SuppressWarnings("all") BuilderSingularToBuilderWithNullBuilder clearElems() {
+      if ((this.elems != null))
+          this.elems.clear();
+      return this;
+    }
+    public @java.lang.SuppressWarnings("all") BuilderSingularToBuilderWithNull build() {
+      java.util.List<String> elems;
+      switch (((this.elems == null) ? 0 : this.elems.size())) {
+      case 0 :
+          elems = java.util.Collections.emptyList();
+          break;
+      case 1 :
+          elems = java.util.Collections.singletonList(this.elems.get(0));
+          break;
+      default :
+          elems = java.util.Collections.unmodifiableList(new java.util.ArrayList<String>(this.elems));
+      }
+      return new BuilderSingularToBuilderWithNull(elems);
+    }
+    public @java.lang.Override @java.lang.SuppressWarnings("all") java.lang.String toString() {
+      return (("BuilderSingularToBuilderWithNull.BuilderSingularToBuilderWithNullBuilder(elems=" + this.elems) + ")");
+    }
+  }
+  private @Singular java.util.List<String> elems;
+  public static void test() {
+    new BuilderSingularToBuilderWithNull(null).toBuilder();
+  }
+  @java.lang.SuppressWarnings("all") BuilderSingularToBuilderWithNull(final java.util.List<String> elems) {
+    super();
+    this.elems = elems;
+  }
+  public static @java.lang.SuppressWarnings("all") BuilderSingularToBuilderWithNullBuilder builder() {
+    return new BuilderSingularToBuilderWithNullBuilder();
+  }
+  public @java.lang.SuppressWarnings("all") BuilderSingularToBuilderWithNullBuilder toBuilder() {
+    return new BuilderSingularToBuilderWithNullBuilder().elems(((this.elems == null) ? java.util.Collections.emptyList() : this.elems));
+  }
+}