fix: add missing nil check

[pd93]

Fixes #961.

This problem occurs when a user uses version: '2' in their Taskfile in combination with a Taskfile_{{OS}}.yml file. In the case of #961, I'm guessing there was only a Taskfile_windows.yml file which is why the issue doesn't repro on Linux.

This happens because the Merge function is called with includedTaskfile set to nil:

task/taskfile/read/taskfile.go

Lines 171 to 182 in 5be1f6b

	if v < 3.0 {
	path = filepathext.SmartJoin(readerNode.Dir, fmt.Sprintf("Taskfile_%s.yml", runtime.GOOS))
	if _, err = os.Stat(path); err == nil {
	osTaskfile, err := readTaskfile(path)
	if err != nil {
	return nil, "", err
	}
	if err = taskfile.Merge(t, osTaskfile, nil); err != nil {
	return nil, "", err
	}
	}
	}

This value is then not being nil checked in the Merge function here:

task/taskfile/merge.go

Line 48 in 5be1f6b

task.Internal = task.Internal || includedTaskfile.Internal

The fix simply adds a nil check to this line.

[andreynering]

Thanks!