Permance improvement: remove IsNull check when get property from feature source

[hubu]

IsNull check has some overhead, especially when the feature source connect to database such as Oracle, SQL Server, and etc. The solution is, catch all exceptions, if the property is indeed null, we will rethrow an MgNullPropertyValueException. The benifit for this solution is, we don't need to do IsNull check when get properties only if the properties is indeed null.

The test results show that this solution can decrease the query featue time about 3-5%. The test data is based on Oracle datasets, including 100 layers.

[hubu]

fix for this ticket