Cannot evaluate null-returning expression in DoTag when failOnUnknownTokens is enabled

[jsokolowska]

If failOnUnknownTokens option is enabled following code fails with UnknownTokenException:

{% set object = {} %}{% do object.update({}) %}

This always happens if an expression executed by do tag is evaluated to null and failOnUnknown is enabled. Method .update always returns null and there are no missing tokens involved so, logically, I'd expect that to work regardless of the setting