In the case of this class:

public class Test {
        void m1(byte[] b) {}

        void m2() {
                Object o;
                m1((byte[])(o = null));
        }
}

javac generates this code for m2:

void m2();
    descriptor: ()V
    flags:
    Code:
      stack=3, locals=2, args_size=1
         0: aload_0
         1: aconst_null
         2: dup
         3: astore_1
         4: checkcast #2 // class "[B"
         7: checkcast #2 // class "[B"
        10: invokevirtual #3 // Method m1:([B)V
        13: return
      LineNumberTable:
        line 6: 0
        line 7: 13

The second checkcast is unnecessary.