MM-31062: Rewrite empty string checks to be more idiomatic (#16587)

https://mattermost.atlassian.net/browse/MM-31062

```release-note
NONE
```

web/handlers.go

@@ -188,7 +188,7 @@ func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 	token, tokenLocation := app.ParseAuthTokenFromRequest(r)
 
-	if len(token) != 0 && tokenLocation != app.TokenLocationCloudHeader {
+	if token != "" && tokenLocation != app.TokenLocationCloudHeader {
 		session, err := c.App.GetSession(token)
 		if err != nil {
 			c.Log.Info("Invalid session", mlog.Err(err))
@@ -210,7 +210,7 @@ func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		}
 
 		h.checkCSRFToken(c, r, token, tokenLocation, session)
-	} else if len(token) != 0 && c.App.Srv().License() != nil && *c.App.Srv().License().Features.Cloud && tokenLocation == app.TokenLocationCloudHeader {
+	} else if token != "" && c.App.Srv().License() != nil && *c.App.Srv().License().Features.Cloud && tokenLocation == app.TokenLocationCloudHeader {
 		// Check to see if this provided token matches our CWS Token
 		session, err := c.App.GetCloudSession(token)
 		if err != nil {

web/saml.go

@@ -38,7 +38,7 @@ func loginWithSaml(c *Context, w http.ResponseWriter, r *http.Request) {
 	relayProps := map[string]string{}
 	relayState := ""
 
-	if len(action) != 0 {
+	if action != "" {
 		relayProps["team_id"] = teamId
 		relayProps["action"] = action
 		if action == model.OAUTH_ACTION_EMAIL_TO_SSO {
@@ -46,7 +46,7 @@ func loginWithSaml(c *Context, w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
-	if len(redirectTo) != 0 {
+	if redirectTo != "" {
 		relayProps["redirect_to"] = redirectTo
 	}