Fix DateTime.from_iso8601/2 on get_repo_popularity/1

[pbrudnick]

[error][2022-11-15 06:47:35.792] Task #PID<0.32181.3> started from #PID<0.32178.3> terminating
** (FunctionClauseError) no function clause matching in Calendar.ISO.parse_utc_datetime/1
(elixir 1.12.3) lib/calendar/iso.ex:510: Calendar.ISO.parse_utc_datetime(nil)
(elixir 1.12.3) lib/calendar/datetime.ex:1073: DateTime.from_iso8601/2
(coophub 0.4.3) lib/coophub/repos.ex:173: Coophub.Repos.get_repo_popularity/1
(coophub 0.4.3) lib/coophub/cache_warmer.ex:231: anonymous fn/1 in Coophub.CacheWarmer.put_popularities/1
(elixir 1.12.3) lib/enum.ex:1582: Enum."-map/2-lists^map/1-0-"/2
(elixir 1.12.3) lib/enum.ex:1582: Enum."-map/2-lists^map/1-0-"/2
(coophub 0.4.3) lib/coophub/cache_warmer.ex:214: Coophub.CacheWarmer.get_org_with_repos/2
(elixir 1.12.3) lib/enum.ex:1582: Enum."-map/2-lists^map/1-0-"/2
Function: #Function<14.67629458/0 in Coophub.CacheWarmer.warm_cache/0>
Args: []