add fallback usdt conversion

[danlipert]

Description

a previous change to a fix for incorrect token prices inadvertently broke conversion rate for some tokens

Refers/Fixes

https://github.com/gitcoinco/web/pull/4792/files

Testing

tested locally

[codecov]

Codecov Report

Merging #4813 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4813      +/-   ##
==========================================
+ Coverage   30.45%   30.46%   +0.01%     
==========================================
  Files         216      216              
  Lines       17313    17316       +3     
  Branches     2350     2350              
==========================================
+ Hits         5272     5275       +3     
  Misses      11832    11832              
  Partials      209      209

Impacted Files	Coverage Δ
app/grants/models.py	57.82% <100%> (+0.4%)	⬆️

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@6d8efdb). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #4813   +/-   ##
=========================================
  Coverage          ?   30.43%           
=========================================
  Files             ?      216           
  Lines             ?    17308           
  Branches          ?     2348           
=========================================
  Hits              ?     5267           
  Misses            ?    11832           
  Partials          ?      209

Impacted Files	Coverage Δ
app/grants/models.py	57.82% <100%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6d8efdb...8fa07f3. Read the comment docs.