Duplication of suggestions with empty attributes

A customer is seeing duplicate suggestions in his index. After investigation, the reason is most likely because of empty attributes that are creating a space and thus not _dedup_.
For example, with this config:
```
a + b
a + b + c
```

Data:
```
{ a: "renault", b: "clio", c: "" }
{ a: "renault", b: "clio", c: "3 portes" }
```

You can end up with this result:

Start by reproducing the duplicate suggestions with the configuration and two data records shown in the issue. Inspect the query-suggestions deduplication behavior around empty attributes; done means equivalent suggestions are not duplicated while distinct values such as “3 portes” remain separate.

Written by the indexing model from the issue text.