Add overload of ping command for nil message style

Author: Mordil

Sources/RediStack/Commands/ConnectionCommands.swift

@@ -41,6 +41,9 @@ extension RedisCommand {
         }
     }
 
+    /// [PING](https://redis.io/commands/ping)
+    public static var ping: RedisCommand<String> { Self.ping(with: nil) }
+
     /// [AUTH](https://redis.io/commands/auth)
     /// - Parameter password: The password to authenticate with.
     public static func auth(with password: String) -> RedisCommand<Void> {

Tests/RediStackIntegrationTests/Commands/ConnectionCommandsTests.swift

@@ -23,6 +23,9 @@ final class ConnectionCommandsTests: RediStackIntegrationTestCase {
 
         let second = try connection.ping(with: "My message").wait()
         XCTAssertEqual(second, "My message")
+
+        let third = try connection.send(.ping).wait()
+        XCTAssertEqual(third, first)
     }
 
     func test_echo() throws {