Protect imdb_id field from rottentomatoes_lookup

If the alternate_ids list in rottentomatoes_lookup's movie object does not contains a valid imdb_id, the plugin **might** assign it to None on target entry, eventually clearing a real value **or** preventing any other metainfo plugin to assign it (since the field it's not lazy anymore).

Example 1 (imdb_id is _None_):
```
  rtl:
    mock:
      - { title: '12 Years a Slave' }
    imdb_lookup: yes
    rottentomatoes_lookup: yes
    if:
      - imdb_languages: accept
      - rt_critics_score: reject
```

Example 2 (changing the conditions order the imdb_id is good):
```
  rtl:
    mock:
      - { title: '12 Years a Slave' }
    imdb_lookup: yes
    rottentomatoes_lookup: yes
    if:
      - rt_critics_score: reject
      - imdb_languages: accept
```