MtApi5: fixed problem of null string in command's parameters

MtApi5/MqlTick.cs

@@ -1,4 +1,6 @@
-namespace MtApi5
+using MtApi5.MtProtocol;
+
+namespace MtApi5
 {
     public class MqlTick
     {
@@ -15,6 +17,19 @@
         {
         }
 
+        internal MqlTick(MtTick? tick)
+        {
+            if (tick != null)
+            {
+                MtTime = tick.Time;
+                bid = tick.Bid;
+                ask = tick.Ask;
+                last = tick.Last;
+                volume = tick.Volume;
+                volume_real = tick.VolumeReal;
+            }
+        }
+
         public long MtTime { get; set; }          // Time of the last prices update
 
         public double bid { get; set; }           // Current Bid price

MtApi5/Mt5Quote.cs

@@ -2,7 +2,7 @@
 {
     public class Mt5Quote
     {
-        public string? Instrument { get; set; }
+        public string Instrument { get; set; } = string.Empty;
         public double Bid { get; set; }
         public double Ask { get; set; }
         public int ExpertHandle { get; set; }