chore: fix sync_amount API + prettify conversion table

[thelostone-mc]

Description

• If ETH/ USDT conversion isn't found -> the value is set to null
• Fixes issue where /sync_amount returns 500 cause conversion rate of ETH was not set

Reported over at https://gitcoincore.slack.com/archives/C013CL18L72/p1593716951050300

Testing

Admin View

[codecov]

Codecov Report

Merging #7049 into master will increase coverage by 0.02%.
The diff coverage is 44.00%.

@@            Coverage Diff             @@
##           master    #7049      +/-   ##
==========================================
+ Coverage   26.34%   26.36%   +0.02%     
==========================================
  Files         300      300              
  Lines       29156    29160       +4     
  Branches     4283     4283              
==========================================
+ Hits         7682     7689       +7     
+ Misses      21204    21200       -4     
- Partials      270      271       +1     

Impacted Files	Coverage Δ
app/dashboard/helpers.py	14.46% <41.66%> (-0.10%)	⬇️
app/economy/admin.py	64.28% <100.00%> (+1.32%)	⬆️
app/dashboard/embed.py	31.60% <0.00%> (+3.44%)	⬆️

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