Enable govet check for empty strings (#16785)

Adds the check for https://github.com/mattermost/mattermost-server/issues/16674
in the Makefile

```release-note
NONE
```

Makefile

@@ -576,7 +576,7 @@ vet: ## Run mattermost go vet specific checks
 		echo "mattermost-govet is not installed. Please install it executing \"GO111MODULE=off GOBIN=$(PWD)/bin go get -u github.com/mattermost/mattermost-govet\""; \
 		exit 1; \
 	fi;
-	@VET_CMD="-license -structuredLogging -inconsistentReceiverName -inconsistentReceiverName.ignore=session_serial_gen.go,team_member_serial_gen.go,user_serial_gen.go -tFatal"; \
+	@VET_CMD="-license -structuredLogging -inconsistentReceiverName -inconsistentReceiverName.ignore=session_serial_gen.go,team_member_serial_gen.go,user_serial_gen.go -emptyStrCmp -tFatal"; \
 	if ! [ -z "${MM_VET_OPENSPEC_PATH}" ] && [ -f "${MM_VET_OPENSPEC_PATH}" ]; then \
 		VET_CMD="$$VET_CMD -openApiSync -openApiSync.spec=$$MM_VET_OPENSPEC_PATH"; \
 	else \

web/saml.go

@@ -107,7 +107,7 @@ func completeSaml(c *Context, w http.ResponseWriter, r *http.Request) {
 	hasRedirectURL := false
 	if val, ok := relayProps["redirect_to"]; ok {
 		redirectURL = val
-		hasRedirectURL = len(val) > 0
+		hasRedirectURL = val != ""
 	}
 
 	handleError := func(err *model.AppError) {