chore: add ut and docs for sidecar injector

Signed-off-by: seeflood <zhou.qunli@foxmail.com>

pkg/injector/patcher/sidecar_patcher_test.go

@@ -97,7 +97,7 @@ func TestGetPatch(t *testing.T) {
 		if err != nil {
 			t.Errorf("Unexpected error: %v", err)
 		}
-		if patch == nil || len(patch) == 0 {
+		if len(patch) == 0 {
 			t.Errorf("Expected patch, got nil or empty")
 		}
 	})