SONAR-12513 last used date is never set on new Quality Profiles

all QP start with no last used date in the Database, which is represented by a null value in the column,
the update method ensures no update is done if new date is older than the current one
it does so by comparing the new date with the current value, however, this test always returns false if the current value is null

this way of updating the last used date was introduced to fix SONAR-10462 (Update of last used date on Quality Profile can fail with Deadlock on Postgres)
since then, last used date of new QP (which includes any new install of SQ since 7.8, including the LTS) is never set