requires_check_nil

[timrulebosch]

For variables without a value, the requires checks do not fail.

In the following taskfile, each variable evaluates to "" (a nil value), and still the required checks pass.

version: '3'
tasks:
  default:
    vars:
      A: null
      B: ''
      C: '{{.C}}'
      D:
        sh: echo ""
    cmds:
      - echo "A={{.A}}"
      - echo "B={{.B}}"
      - echo "C={{.C}}"
      - echo "D={{.D}}"
    requires:
      vars: [A, B, C, D]

This PR introduces a check for "nil" values which corrects this behavior.

Additional notes:
In RunTask the call sequence is very sensitive. areTaskRequiredVarsSet depends on dynamic variables being resolved, which happens in CompiledTask (a later call), however shouldRunOnCurrentPlatform (an earlier call) must occur before dynamic variables are expanded.

The check for areTaskRequiredVarsSet occurs between those two calls (FastCompiledTask and CompiledTask). Therefore it does not have access to resolved dynamic variables. Moving calls around causes tests to fail, indicating intricate dependencies.

Solution was to add a call FastCompiledTaskWithVars to the sequence which resolves the dynamic vars. Those dynamic vars should be cached, based on my code analysis, and therefore I don't expect any noticeable performance hit.

fixes #2243