Fix support of non required values in qgisform
Fix support of non required values in qgisform

If the expected value is an integer, but '' is given and the value
is not required, the final value should be null, and not 0, else it
can fail in some case. For example, if the value is a foreign key,
the id 0 may not exists in the foreign table.

nboisteault/lizmap-web-client,3liz/lizmap-web-client,rldhont/lizmap-web-client,nboisteault/lizmap-web-client,3liz/lizmap-web-client,laurentj/lizmap-web-client,laurentj/lizmap-web-client,rldhont/lizmap-web-client,nboisteault/lizmap-web-client,rldhont/lizmap-web-client,3liz/lizmap-web-client,laurentj/lizmap-web-client,3liz/lizmap-web-client,nboisteault/lizmap-web-client,rldhont/lizmap-web-client,laurentj/lizmap-web-client