FieldCalculationMap should include a fallback option if parsing failed

[sbouchexbellomie-Philips]

I have a field that used to contain a non integer value in a revision and the FieldCalculationMap failed because FieldCalculationMap::TryConvertToNumeric failed to parse the value.

Proposal: Add a fallback option if parsing failed

• Leave untouched (default)
• Reset to 0

[MrHinsh]

I like... feel free to submit....

Im working on https://github.com/nkdAgility/azure-devops-migration-platform/ and will inclide it there.