fix: special variable type errors in vars with no task context

[pd93]

Fixes #2106

Also related to:

• fix: special variables are defined with dotenv at task level #1810

The fundamental issue is that some special variables are not available when evaluating variables outside of a task. This causes the error seen in #2106 and has actually been an issue for a while. However, it was only surfaced in 3.42 because of the refactor in #2084 which moved a call to GetTaskfileVariables without moving the corresponding if len(tf.Dotenv) == 0 {...} check.

This PR fixes both of these issues:

• Special variables that we are not able to calculate now evaluate to an empty string instead of nil. This avoids type errors like the one seen in Version 3.42 breaks some variable usages inside go templates #2106.
• The validation check for dotenv files has been moved to before the call to GetTaskfileVariables again.