Maybe bug: Sources passed via `:in` don't get recognized in some cases

[NPException]

I was writing a query with an aggretate function wher I wanted to provide an optional parameter to further limit the query results by.
The query did not return the expected results and I'm lead to believe that this might be a bug in Asami.

Steps to reproduce:

• Create an in-memory database.
• Add the following data:

  (d/transact conn
    [{:name "Carl" :city "Berlin"}
     {:name "Tom" :city "Bern"}
     {:name "Claire" :city "Paris"}
     {:name "Sofia" :city "Berlin"}])

• Query how many people live in each city

  (d/q '[:find ?city (count ?name)
         :where (and
                  [?a :city ?city]
                  [?a :name ?name])]
    db)

• Result is as expected: `=> (("Berlin" 2) ("Bern" 1) ("Paris" 1))``
• Query but with an optional parameter to limit by city:

  (d/q '[:find ?city (count ?name)
         :in $ ?only-city
         :where (and
                  [?a :city ?city]
                  (or [(empty? ?only-city)]
                      [?a :city ?only-city])
                  [?a :name ?name])]
    db
    "Berlin")

• Instead of => (("Berlin" 2)), the result is an error:

  Execution error (ExceptionInfo) at asami.query/eval6873$filter-join$fn (query.cljc:240).
  Unknown variable in filter: ?only-city
  

Using nil? instead of empty? in the filter just results in the same result as the first query.

Even if I make the parameter mandatory by removing the filter, the result is not the expected 2 for "Berlin":

(d/q '[:find (count ?name) .
       :in $ ?only-city
       :where (and
                [?a :city ?only-city]
                [?a :name ?name])]
  db
  "Berlin")
=> 4

Do I not understand how the aggregate queries are supposed to work, or is this actually a bug in Asami?