This PR is a followup on https://github.com/odoo/enterprise/pull/27676 where a fix was set in stable to manage the specific case of empty placeholders. However this fix was not stable and made pre-existing default placeholder disappear and it was hence revert in https://github.com/odoo/enterprise/pull/28496 on stable versions. This fix in master completes the first fix to treat other issues like the case where the user is not in developer mode, the false that appear in the situation of empty placeholders and a confusion in the js logic where we could not differentiate the name of a sign item that was just created (and has a default value of false) and a sign item that had its placeholder set to false. We also add the placeholders in the demo data. The commits of this PR totally rewrite the first fix commit and can be seen as a minor behavior change (the name field of the sign item always has the value of its placeholder while previously it was only the placeholder when it was not false, this mean that changing the type placeholder won't change the previously existing item default placeholder like it was the case before).

BUG1

Steps to reproduce:
- Install sign
- Put no developer mode on
- Set a signature block on a document
- Change if from customer to employee

Current behavior:
The block shows false

Expected behavior:
The block does not change

Explanation:
The bug was created after the fix commit 860ace5013d7f5cde1018f5eb1ec4fce1f6b3537
the fix did not take into consideration that the o_sign_name of the block form
only appeared when the use is in developer mode (t-if="widget.debug" in xml). 
https://github.com/odoo/enterprise/blob/da1b8691f8edceac5a7ba794921a24657886618c/sign/static/src/xml/sign_backend.xml#L5-L9
So the name was always false when pulled in the form hide function. To fix that we only pull
the name from o_sign_name if we are not in developer mode.

BUG2

Steps to reproduce:

- Install sign
- Create a template and add a sign item
- Set an empty placeholder with the developer mode
on the sign item and save

Expected behavior:
The placeholder is empty

Current behavior:
The placeholder is false

Explanation:
It was thought in a previous fix https://github.com/odoo/enterprise/commit/860ace5013d7f5cde1018f5eb1ec4fce1f6b3537
that the false value was the expected behavior. But is was just inudced by
an empty placeholder in the sign item type that returned the false value.
To solve the issue we add || "" after the different places where the name
is set so that if the type placeholder is false then the sign item
placeholder is an empty string.

BUG3
Steps to reproduce:
- Create a sign item
- Set an empty placeholder with developer mode
- Refresh
- Click on the sign item you just created

Expected behavior:
The placeholder is empty

Current behavior:
The placeholder is the sign item type default placeholder

Explanation:
The javascript is unabled to make the difference between the
name of a sign item that was just created (and has a default
value of false) and a sign item that had its placeholder set
to false. We solve that in the backend by setting a default
value for the sign item name at creation. It's cleaner than
the current implementation as the default placeholder is only
set once and it makes more sense.

opw-2767417
opw-2877149