fix xerrors.Is check

Author: asmyasnikov

internal/xerrors/xerrors.go

@@ -79,9 +79,6 @@ func As(err error, targets ...interface{}) bool {
 // Is is a improved proxy to errors.Is
 // This need to single import errors
 func Is(err error, targets ...error) bool {
-	if err == nil {
-		panic("nil err")
-	}
 	if len(targets) == 0 {
 		panic("empty targets")
 	}