[Form] Renamed the option "empty_value" to "placeholder"

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #5791
| License       | MIT
| Doc PR        | TODO

This PR is changing the "empty_value" option to the more understandable name "placeholder".

In a subsequent PR, the "placeholder" option should also be added to all types that support the "placeholder" HTML5 attribute.