ValueViewModel: react to FormattedValue = null if no _partialUserInput was used

In simple cases, e.g. when using the VM programmatically, _partialUserInput
is always null. When the Valueis not null and FormattedValue is assigned
null, this lead to wrongly ignoring the input.

Depending on the current state we have to avoid looking at
_partialUserInput.
nuspec/clientaspnet: make include exists check more robust