Pull request: 2546 updater fix

Merge in DNS/adguard-home from 2546-updater-fix to master

Closes #2546.

Squashed commit of the following:

commit af243c9fad710efe099506fda281e628c3e5ec30
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Jan 13 14:33:37 2021 +0300

    updater: fix go 1.14 compat

commit 742fba24b300ce51c04acb586996c3c75e56ea20
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Jan 13 13:58:27 2021 +0300

    util: imp error check

commit c2bdbce8af657a7f4b7e05c018cfacba86e06753
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Jan 11 18:51:26 2021 +0300

    all: fix and refactor update checking

HACKING.md

@@ -46,6 +46,14 @@ The rules are mostly sorted in the alphabetical order.
 
  *  Avoid `new`, especially with structs.
 
+ *  Check against empty strings like this:
+
+    ```go
+    if s == "" {
+            // …
+    }
+    ```
+
  *  Constructors should validate their arguments and return meaningful errors.
     As a corollary, avoid lazy initialization.