Meta writer full_file_path returns null when acquisition first created

[GDYendell]

This would be OK if metadata was provided, but without it fastcs-odin relies on the value to infer the type and has no way to tell it is a str.

To reproduce, set the acquisition id to create an acquisition and then:

curl -H 'Accept: application/json;metadata=True' 127.0.0.1:8888/api/0.1/mw | jq .
{
...
  "0": {
    "status": {
      "acquisitions": {
        "test": {
          "full_file_path": null,
          "num_processors": 0,
          "written": 0,
          "writing": false
        }
      }
...

Update to return empty string