Disallow casting null to string

Author: xPaw

ValveKeyValue/ValveKeyValue/KVObject_IConvertible.cs

@@ -190,9 +190,9 @@ KVValueType.Collection or KVValueType.Array or KVValueType.BinaryBlob
         /// <inheritdoc cref="IConvertible.ToString"/>
         public string ToString(IFormatProvider provider) => ValueType switch
         {
-            KVValueType.String => (string)_ref ?? string.Empty,
+            KVValueType.String => (string)_ref,
             KVValueType.Boolean => _scalar != 0 ? "1" : "0",
-            KVValueType.Null => string.Empty,
+            KVValueType.Null => throw new NotSupportedException("Cannot convert null to String."),
             KVValueType.FloatingPoint => BitConverter.Int32BitsToSingle((int)_scalar).ToString(provider),
             KVValueType.FloatingPoint64 => BitConverter.Int64BitsToDouble(_scalar).ToString(provider),
             KVValueType.Int32 or KVValueType.Pointer => ((int)_scalar).ToString(provider),

ValveKeyValue/ValveKeyValue/Serialization/KeyValues1/KV1TextSerializer.cs

@@ -107,7 +107,11 @@ void WriteKeyValuePair(string name, KVObject value)
             WriteText(name);
             writer.Write('\t');
 
-            if (value.ValueType == KVValueType.Boolean)
+            if (value.IsNull)
+            {
+                WriteText(string.Empty);
+            }
+            else if (value.ValueType == KVValueType.Boolean)
             {
                 WriteText(value.ToBoolean(null) ? "1" : "0");
             }