Yes, it's a rule of standard Korean pronunciation. You may wonder why. Because 끝소리 ㅇ[ŋ] and ㄴ[n] are both nasal sounds(moving air through your nose), ㄴ is easier to pronounce right after 끝소리 ㅇ than ㄹ(you don't use your nose) is. For example, pronounce "sing-nee" and "sing-lee". Which one is easier?