Throw InvalidCastException when GetGuid fails.

This is a breaking API change from Connector/NET, but matches other ADO.NET connectors.

Author: bgrainger

src/MySqlConnector/Core/Row.cs

@@ -141,9 +141,7 @@ public Guid GetGuid(int ordinal)
 			if (value is byte[] bytes && bytes.Length == 16)
 				return new Guid(bytes);
 
-			if (m_dataOffsets[ordinal] == -1)
-				throw new InvalidCastException("Column is NULL.");
-			throw new MySqlException("The value could not be converted to a GUID: {0}".FormatInvariant(value));
+			throw new InvalidCastException("The value could not be converted to a GUID: {0}".FormatInvariant(value));
 		}
 
 		public short GetInt16(int ordinal)

tests/SideBySide/DataTypes.cs

@@ -394,7 +394,7 @@ public async Task GetGuid(string column, Type fieldType)
 					Assert.NotNull(reader.GetValue(0));
 					Assert.IsType(fieldType, reader.GetValue(0));
 
-					Type exceptionType = typeof(MySqlException);
+					Type exceptionType = typeof(GetGuidWhenNullException);
 #if BASELINE
 					// baseline throws FormatException when conversion from string fails
 					if (fieldType == typeof(string))