save empty curve if no contributions to fix sorting

[danlipert]

Description

This PR sets an empty clr prediction curve for grants that have no contributions, so that sorting on the grants index page works as expected

Refers/Fixes

Testing

Tested this fix in interactive shell on production

[codecov]

Codecov Report

Merging #5739 into master will increase coverage by 0.93%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5739      +/-   ##
==========================================
+ Coverage   30.01%   30.95%   +0.93%     
==========================================
  Files         254      253       -1     
  Lines       21591    22064     +473     
  Branches     3138     3267     +129     
==========================================
+ Hits         6481     6830     +349     
- Misses      14834    14936     +102     
- Partials      276      298      +22

Impacted Files	Coverage Δ
app/grants/views.py	14.09% <0%> (-0.54%)	⬇️
...arketing/management/commands/new_bounties_email.py	47.05% <0%> (ø)	⬆️
app/dashboard/views.py	12.24% <0%> (ø)	⬆️
...rketing/management/commands/no_applicants_email.py	0% <0%> (ø)	⬆️
app/app/settings.py	81.33% <0%> (+1.12%)	⬆️
app/grants/models.py	61.48% <0%> (+4.28%)	⬆️
app/dashboard/utils.py	48.68% <0%> (+7.91%)	⬆️

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