[ASTERIXDB-3015][FUN] Fix avg() handling of first non-number

[ASTERIXDB-3015][FUN] Fix avg() handling of first non-number

- user model changes: no
- storage format changes: no
- interface changes: no

Details:
- Fix incorrect result produced by avg() function
  when its first input value is not a number

Change-Id: I21cc52c56fb79a609aabdf994a7b6e19d570b6ea
Reviewed-on: https://asterix-gerrit.ics.uci.edu/c/asterixdb/+/15305
Integration-Tests: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Tested-by: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Reviewed-by: Dmitry Lychagin <dmitry.lychagin@couchbase.com>
Reviewed-by: Ali Alsuliman <ali.al.solaiman@gmail.com>