Fix error that occurs when querying intervals for missing chr

[athos]

When issuing an interval query (using c.u.i/find-overlap-intervals) for a missing chr in the index, an error will be thrown:

(require '[cljam.util.intervals :as intervals])

(def index
  (intervals/index-intervals
   [{:chr "chr1", :start 10000, :end 15000}
    {:chr "chr1", :start 20000, :end 25000}]))

(intervals/find-overlap-intervals index "chr1" 13000 17000)
;=> ({:chr "chr1", :start 10000, :end 15000})
(intervals/find-overlap-intervals index "chr1" 17000 19000)
;=> ()

(intervals/find-overlap-intervals index "chr2" 13000 17000)
;; Execution error (IllegalArgumentException) at cljam.util.intervals/eval5662$fn$G (intervals.clj:4).
;; No implementation of method: :find-overlap-intervals* of protocol: #'cljam.util.intervals/IIntervals found for class: nil

This PR fixes the error by guarding against nil with some->.

[alumi]

Wow, thank you for the fix! 👍

[niyarin]

Thank you for fixing. LGTM 👍
(I wrote the original code , but I overlooked the case with no corresponding chr.)

[athos]

@alumi @niyarin Thanks for the quick review!

@alumi Do you think it's worth cutting another release soon?

[alumi]

Sure! I'll release 0.8.2 in an hour 👍

[athos]

Thanks a lot 🙏 I'm looking forward to it!