[FIX] web: remove min-height of form sheet within a modal

We added min-height for `o_form_sheet` in form view with a commit[1] restoring
the views design (after views refactoring).

However, the min-height isn't very useful when form view is opened witin a
modal (FormViewDialog). Especially, when the form view is having only few
fields, the `o_form_sheet` container occupies unnecessary height and looks
a bit ugly.

This commit fixes the issues by setting the min-height to `0` for
`o_form_sheet` container in such cases.

[1] - https://github.com/odoo/odoo/commit/250e716c3a12#diff-01e46b3171a7282967199e5d45fca0e6R15



TaskID: 2150471
PR #41619

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>