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

test/transform/resource/before/BuilderSingularToBuilderWithNull.java

@@ -0,0 +1,10 @@
+import lombok.Singular;
+
+@lombok.Builder(toBuilder = true)
+class BuilderSingularToBuilderWithNull {
+	@Singular private java.util.List<String> elems;
+	
+	public static void test() {
+		new BuilderSingularToBuilderWithNull(null).toBuilder();
+	}
+}