[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#101042

X-original-commit: 5976e19a31b9b0edaefc1079541514810d12b240
Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>

addons/web/static/src/views/fields/many2many_tags/many2many_tags_field.xml

@@ -10,7 +10,7 @@
             <div t-if="showM2OSelectionField" class="o_field_many2many_selection d-inline-flex w-100" t-on-keydown="onAutoCompleteKeydown" t-ref="autoComplete">
                 <Many2XAutocomplete
                     id="props.id"
-                    placeholder="props.placeholder"
+                    placeholder="tags.length ? '' : props.placeholder"
                     resModel="props.relation"
                     autoSelect="true"
                     fieldString="string"

addons/web/static/src/views/fields/many2many_tags_avatar/many2many_tags_avatar_field.xml

@@ -10,7 +10,7 @@
             <div t-if="showM2OSelectionField" class="o_field_many2many_selection d-inline-flex w-100" t-on-keydown="onAutoCompleteKeydown" t-ref="autoComplete">
                 <Many2XAutocomplete
                     id="props.id"
-                    placeholder="props.placeholder"
+                    placeholder="tags.length ? '' : props.placeholder"
                     resModel="props.relation"
                     autoSelect="true"
                     fieldString="props.string"

addons/web/static/tests/views/fields/many2many_tags_field_tests.js

@@ -1585,6 +1585,13 @@ QUnit.module("Fields", (hooks) => {
             target.querySelector(".o_field_widget[name='timmy'] input").placeholder,
             "Placeholder"
         );
+
+        await selectDropdownItem(target, "timmy", "gold");
+
+        assert.strictEqual(
+            target.querySelector(".o_field_widget[name='timmy'] input").placeholder,
+            ""
+        );
     });
 
     QUnit.test("Many2ManyTagsField supports 'create' props to be a Boolean", async (assert) => {