Update source3.cs

[spence-m]

Title

Updated source3.cs to check the array length is zero, instead of checking for null

Summary

GetCustomAttributes never returns null, but instead returns an empty array when no attributes are found, as outlined here: https://msdn.microsoft.com/en-us/library/ms130871(v=vs.110).aspx. I have updated the docs to check that the array length is zero instead of checking for null.

[rpetrusha]

Thanks for correcting this programming error, @matthewspencerboxuk, and for contributing to the dotnet/docs repo. There are also Visual Basic and C++ versions of this example (at samples/snippets/visualbasic/VS_Snippets_CLR/conceptual.attributes.usage/vb/source3.vb and samples/snippets/cpp/VS_Snippets_CLR/conceptual.attributes.usage\cpp/source3.cpp. Could you make this change there as well? We'd also be happy to make the change, If you'd prefer.

[spence-m]

Hey @rpetrusha, happy to make the VB and C++ changes, will have a look into this later today.

[rpetrusha]

Thanks for making this correction, @matthewspencerboxuk. We'll merge your PR now and open a separate PR to change the Visual Basic and C++ code.