Update EmbedFieldBuilder

Updated Field builder, to allow for "Empty" fields and values.

If you send "_ _" via the discord rest API, it will display as a blank field.

Updated the Value setting to use String.IsNullOrWhiteSpace instead of String.IsNullOrEmpty, because passing a string full of spaces through the API will cause errors as well. However, "_ _" works perfectly.

src/Discord.Net.Core/Entities/Messages/EmbedBuilder.cs

@@ -239,9 +239,12 @@ namespace Discord
             get => _name;
             set
             {
-                if (string.IsNullOrWhiteSpace(value)) throw new ArgumentException($"Field name must not be null, empty or entirely whitespace.", nameof(Name));
-                if (value.Length > MaxFieldNameLength) throw new ArgumentException($"Field name length must be less than or equal to {MaxFieldNameLength}.", nameof(Name));
-                _name = value;
+                if (string.IsNullOrWhiteSpace(value))
+                    _name = "_ _";
+                else if (value.Length > MaxFieldNameLength)
+                    throw new ArgumentException($"Field name length must be less than or equal to {MaxFieldNameLength}.", nameof(Name));
+                else
+                    _name = value;
             }
         }
 
@@ -251,9 +254,12 @@ namespace Discord
             set
             {
                 var stringValue = value?.ToString();
-                if (string.IsNullOrEmpty(stringValue)) throw new ArgumentException($"Field value must not be null or empty.", nameof(Value));
-                if (stringValue.Length > MaxFieldValueLength) throw new ArgumentException($"Field value length must be less than or equal to {MaxFieldValueLength}.", nameof(Value));
-                _value = stringValue;
+                if (string.IsNullOrWhiteSpace(stringValue)) 
+                    _value = "_ _";
+                else if (stringValue.Length > MaxFieldValueLength)
+                    throw new ArgumentException($"Field value length must be less than or equal to {MaxFieldValueLength}.", nameof(Value));
+                else 
+                    _value = stringValue;
             }
         }
         public bool IsInline { get; set; }