Bug: Dynamic assignment should allow a blank MAX_CONTACTS_PER_TEXTER variable but does not

[ibrand]

Describe the bug
The intended usage of the env var MAX_CONTACTS_PER_TEXTER was to allow texters to choose as many texts as they'd like if the var was left blank. Instead leaving the var blank makes dynamic assign no longer an option.

To Reproduce
Steps to reproduce the behavior:

1. Set MAX_CONTACTS_PER_TEXTER either to nothing or to 0
2. See that dynamic assignment is no longer available

Expected behavior
We expect that if MAX_CONTACTS_PER_TEXTER is blank, a texter can text as many texts as they'd like and if MAX_CONTACTS_PER_TEXTER is 0, dynamic assign is off.

Additional context
This was first surfaced in #1066 and so the context of that discovery process is there.

[ibrand]

RFC #1533 would impact dynamic assign in a way where this might not be an issue anymore, but that's unclear

[schuyler1d]

I believe this is resolved, but we should confirm. Note -- setting max contacts for a specific assignment to 0 should stop that texter from texting. Setting MAX_CONTACTS env var to 0 doesn't make sense -- that would block all texters from texting. Setting it to blank means there is no maximum.

[schuyler1d]

Realizing this shouldn't be an issue after the rewrite -- max contacts relates to dynamic assignment very differently now