Fix priority bug with taskwarrior<->caldav tool

[twistercool]

Description

Fixes a bug where creating/updating a taskwarrior item with no priority creates an empty PRIORITY: caldav entry.

Also continues syncing remaining elements even if one synchronisation fails instead of exiting immediately.

Fixes # (issue)

#139 and #142

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Test A

Sync tasks taskwarrior and caldav, with some taskwarrior tasks having priority, some without.

Previous version: generates Priority:None item in caldav.
Current version: works

• Test B

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules