[FIX] web: tags field placeholder when tag selected

This commit properly removes the Many2ManyTagsField and
Many2ManyTagsAvatarField's placeholder when at least one tag has been
selected.

Steps to reproduce:
- In Contacts, open a partner's form view
- Edit and set at least one "tag"
- The placeholder is present next to the selected tags

Note: the placeholder was already present in previous versions.
Reported by FP.

closes odoo/odoo#100838

Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>