Updated:
As pointed out in the comments, setting the description to null indicates that the image is purely decorative and is understood as that by screen readers like TalkBack.
Old answer, I no longer support this answer:
For all the people looking how to avoid the warning:
I don't think android:contentDescription="@null"
is the best solution.
I'm using tools:ignore="ContentDescription"
that is what is meant to be.
Make sure you include xmlns:tools="http://schemas.android.com/tools"
in your root layout.