option for trigger formulas to trigger only on changes to existing records

[aeshna-cyanea]

Describe the problem to be solved

currently, trigger formulas always proc on new records, no way to untick the "trigger on new" checkbox when selecting triggers. Alternatively, fix it so that the formula only triggers only if the new record has non-empty columns that the formula is watching.

Describe the solution you would like

UI change to let us disable new record triggers, or a fix to disable triggers when there are no matching columns in the newly created record