Pull request: all: fix client upstreams, imp code

Updates #3186.

Squashed commit of the following:

commit a8dd0e2cda3039839d069fe71a5bd0f9635ec064
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri May 28 12:54:07 2021 +0300

    all: imp code, names

commit 98f86c21ae23b665095075feb4a59dcfcc622bc7
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu May 27 21:11:37 2021 +0300

    all: fix client upstreams, imp code

internal/aghstrings/strings_test.go

@@ -36,6 +36,14 @@ func TestCloneSlice_family(t *testing.T) {
 	})
 }
 
+func TestCoalesce(t *testing.T) {
+	assert.Equal(t, "", Coalesce())
+	assert.Equal(t, "a", Coalesce("a"))
+	assert.Equal(t, "a", Coalesce("", "a"))
+	assert.Equal(t, "a", Coalesce("a", ""))
+	assert.Equal(t, "a", Coalesce("a", "b"))
+}
+
 func TestFilterOut(t *testing.T) {
 	strs := []string{
 		"1.2.3.4",