Fix potential nullref in embedBuilder value setter (#734)

* Fix potential nullref in embedBuilder value setter

* Null check on footer iconUrl

* Adding checks for the other URL properties

* Adding IsNullOrUri extension

* Setting StringExtensions as internal

src/Discord.Net.Core/Extensions/StringExtensions.cs

@@ -0,0 +1,10 @@
+using System;
+
+namespace Discord
+{
+    internal static class StringExtensions
+    {
+        public static bool IsNullOrUri(this string url) =>
+            string.IsNullOrEmpty(url) || Uri.IsWellFormedUriString(url, UriKind.Absolute);
+    }
+}