Unknown entries in contexts should resolve as ''

Current situation

In an expression, accessing an unknown entry in a context raises an exception. For exemple, assuming the following declaration:

variables:
  FOO: This is FOO

The following expression works: ${{ variables.FOO }} but this one fails: ${{ variables.BAR }}.

Desired outcome

To match GitHub Action's behavior and Linux behavior too, ${{ variables.BAR }} should evaluate to an empty string ''.

This should only occurs for the last part of a path. ${{ needs.unknown.result }} should still raise an exception if the current job does not depends on unknown.