ParameterNotNullable should detect null checks in commons-lang StringUtils

[zman0900]

What version of Error Prone are you using?

2.3.1

Does this issue reproduce with the latest release?

2.3.1 is latest

What did you do?

• -Xep:ParameterNotNullable:WARN

import org.apache.commons.lang3.StringUtils;
import javax.annotation.Nullable;

public class MyClass {
    @Nullable
    public Character doTheThing(@Nullable final String str) {
        Character result = null;
        if (StringUtils.isNotEmpty(str)) {
            result = str.charAt(0);
        }
        return result;
    }
}

What did you expect to see?

No warning.

What did you see instead?

[ParameterNotNullable] Nullable parameter not checked for null.

---

It would be useful if ParameterNotNullable could detect the null checking that happens inside this call to StringUtils#isNotEmpty and other similar method inside commons-lang, or maybe even other libraries.