`null` is treated as String?

For , s = s + "hai"; Internally, String.valueOf(null) will be called. That method will convert null to "null".

Byte code :

public static void main(java.lang.String[])   throws java.lang.Exception;
    0:   aconst_null
  //some code
   10:  invokestatic    #27; //Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;// check this line
// some other code
       27:  return

No it wont.Compiler replaces String concatenation with StringBuilder operations which doesn't give null pointer exception.Compiler does all this backend and saves our time of writing boiler-plate code.Actually there is no such thing as String concatenation as String is immutable

Decompiled code:-

    String s = null; 
    s = (new StringBuilder(String.valueOf(s))).append("hai").toString(); 
    System.out.println(s);

So,the answer to your question.

1. Compiler internally changes + concatenation operations using StringBuilder and String.valueOf operations.So the compiler makes sure that null cases would be handled
2. While using a . operator on a String instance,you invoke methods defined in a String instance such as String.concat(String str)which will give a NullPointerException if your String instance is null