How to tell IDEA/Studio that the null check has been done?

You can look into the link that Peter Gromov mention in his answer.

Created some simple classes that resemble your setup:

A class with a method annotated with @Nullable:

The TextUtil class with it's isEmpty method:

And finally the main class calling the TextUtil#isEmpty:

Now if you enter the File -> Settings... and go to Inspections ->Constant conditions & exceptions part you can change the Configure Assert/Check Methods to cater for your isEmpty method:

Add a new IsNull check method:

Enter the TextUtil class, isEmpty method and CharSequence parameter:

This gives this Assert/Check Method Configuration window:

Press Ok and then Ok again to go back to the editor view and you'll see that the inspection disappeared:

You are actually telling IntelliJ that the isEmpty method is doing a null check on the str parameter.

You could use //noinspection ConstantConditions that will remove the NPE warning for the following line, like this:

String encoding = contentEncoding == null ? null : contentEncoding.getValue();

//noinspection ConstantConditions
if (!TextUtils.isEmpty(encoding) && encoding.equalsIgnoreCase("gzip")) {
    inputStream = new GZIPInputStream(entity.getContent());
} else {
    inputStream = entity.getContent();
}