errors: omit redundant nil check in type assertion for Join

When ok is true, err can't be nil.

Make it behave more like the Unwrap function.

Change-Id: Ieba5de57d60f5ff4d6a3468d703e6f72be02a97d
GitHub-Last-Rev: 6df9365
GitHub-Pull-Request: #74764
Reviewed-on: https://go-review.googlesource.com/c/go/+/689920
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Jorropo <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Mark Freeman <[email protected]>
Reviewed-by: Jorropo <[email protected]>

Author: callthingsoff

src/errors/join.go

@@ -28,12 +28,10 @@ func Join(errs ...error) error {
 	}
 	if n == 1 {
 		for _, err := range errs {
-			if err != nil {
-				if _, ok := err.(interface {
-					Unwrap() []error
-				}); ok {
-					return err
-				}
+			if _, ok := err.(interface {
+				Unwrap() []error
+			}); ok {
+				return err
 			}
 		}
 	}