Adding one more condition so that we can override adding shims for net471 in case they are required

[joperezr]

cc: @livarcocc @AlexGhiondea @weshaggard @dsplaisted

During my testing I found one more thing that we can improve with the current targets. The problem to solve, is that in the case your app is in a state where we don't automatically detect that you need the shims (for example if you don't directly reference a netstandard based library but one of your dependencies does) and you are targeting .NET 4.7.1, there is no quick property that you can set in order to force the shims to be deployed with the app. The only current workaround for that, would be to add a direct reference to a netstandard based component directly, so that our logic kicks in. I don't think that this is a good behavior, which is why I would like to try to squish this change in, so that if users need to deploy the shims for 4.7.1, they can simpy set property <DependsOnNETStandard>true</DependsOnNETStandard> in order to accomplish that.

[joperezr]

@dotnet-bot test OSX10.12 Debug please
@dotnet-bot test OSX10.12 Release please (both build succeeded but there was a problem after build was done.)

[livarcocc]

I wonder if this condition should check against "not true" instead of empty. Though, I see that above we are doing the check in a similar manner.

[joperezr]

What we are really checking here is whether or not we have to run the DependsOnNETStandard task, and so what we check is that if we don't know yet if we depend on it or not (if value is empty) then we might want to run the task.

[joperezr]

if we added a condition to be "not true", then what if we want to force the property to be false? We would evaluate this condition to be true and run the task anyway which will override the value we wanted to force, which is why we should check against empty.

[livarcocc]

Could we add some tests for this property?

[joperezr]

I added a test and ran it locally (I do have 4.7.1 targeting pack installed) and it passed successfully.