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

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

@@ -0,0 +1,63 @@
+class BuilderSingularToBuilderWithNull {
+	private java.util.List<String> elems;
+	public static void test() {
+		new BuilderSingularToBuilderWithNull(null).toBuilder();
+	}
+	@java.lang.SuppressWarnings("all")
+	BuilderSingularToBuilderWithNull(final java.util.List<String> elems) {
+		this.elems = elems;
+	}
+	@java.lang.SuppressWarnings("all")
+	public static class BuilderSingularToBuilderWithNullBuilder {
+		@java.lang.SuppressWarnings("all")
+		private java.util.ArrayList<String> elems;
+		@java.lang.SuppressWarnings("all")
+		BuilderSingularToBuilderWithNullBuilder() {
+		}
+		@java.lang.SuppressWarnings("all")
+		public BuilderSingularToBuilderWithNullBuilder elem(final String elem) {
+			if (this.elems == null) this.elems = new java.util.ArrayList<String>();
+			this.elems.add(elem);
+			return this;
+		}
+		@java.lang.SuppressWarnings("all")
+		public BuilderSingularToBuilderWithNullBuilder elems(final java.util.Collection<? extends String> elems) {
+			if (this.elems == null) this.elems = new java.util.ArrayList<String>();
+			this.elems.addAll(elems);
+			return this;
+		}
+		@java.lang.SuppressWarnings("all")
+		public BuilderSingularToBuilderWithNullBuilder clearElems() {
+			if (this.elems != null) this.elems.clear();
+			return this;
+		}
+		@java.lang.SuppressWarnings("all")
+		public 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);
+		}
+		@java.lang.Override
+		@java.lang.SuppressWarnings("all")
+		public java.lang.String toString() {
+			return "BuilderSingularToBuilderWithNull.BuilderSingularToBuilderWithNullBuilder(elems=" + this.elems + ")";
+		}
+	}
+	@java.lang.SuppressWarnings("all")
+	public static BuilderSingularToBuilderWithNullBuilder builder() {
+		return new BuilderSingularToBuilderWithNullBuilder();
+	}
+	@java.lang.SuppressWarnings("all")
+	public BuilderSingularToBuilderWithNullBuilder toBuilder() {
+		return new BuilderSingularToBuilderWithNullBuilder().elems(this.elems == null ? java.util.Collections.emptyList() : this.elems);
+	}
+}