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

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

```release-note
NONE
```

api4/user.go

@@ -559,7 +559,7 @@ func getFilteredUsersStats(c *Context, w http.ResponseWriter, r *http.Request) {
 		}
 	}
 	channelRoles := []string{}
-	if channelRolesString != "" && len(channelID) != 0 {
+	if channelRolesString != "" && channelID != "" {
 		channelRoles, rolesValid = model.CleanRoleNames(strings.Split(channelRolesString, ","))
 		if !rolesValid {
 			c.SetInvalidParam("channelRoles")
@@ -567,7 +567,7 @@ func getFilteredUsersStats(c *Context, w http.ResponseWriter, r *http.Request) {
 		}
 	}
 	teamRoles := []string{}
-	if teamRolesString != "" && len(teamID) != 0 {
+	if teamRolesString != "" && teamID != "" {
 		teamRoles, rolesValid = model.CleanRoleNames(strings.Split(teamRolesString, ","))
 		if !rolesValid {
 			c.SetInvalidParam("teamRoles")
@@ -673,7 +673,7 @@ func getUsers(c *Context, w http.ResponseWriter, r *http.Request) {
 		}
 	}
 	channelRoles := []string{}
-	if channelRolesString != "" && len(inChannelId) != 0 {
+	if channelRolesString != "" && inChannelId != "" {
 		channelRoles, rolesValid = model.CleanRoleNames(strings.Split(channelRolesString, ","))
 		if !rolesValid {
 			c.SetInvalidParam("channelRoles")
@@ -681,7 +681,7 @@ func getUsers(c *Context, w http.ResponseWriter, r *http.Request) {
 		}
 	}
 	teamRoles := []string{}
-	if teamRolesString != "" && len(inTeamId) != 0 {
+	if teamRolesString != "" && inTeamId != "" {
 		teamRoles, rolesValid = model.CleanRoleNames(strings.Split(teamRolesString, ","))
 		if !rolesValid {
 			c.SetInvalidParam("teamRoles")