UX: Multichain: Fix useAccountTotalFiatBalance when getWeiHexFromDecimalValue returns "NaN"

[darkwing]

Description

While onboarding with MULTICHAIN environment variable turned on, I noticed a crash upon initial loading screen. The cause was getWeiHexFromDecimalValue returning "NaN" in this block:

const totalWeiBalance = getWeiHexFromDecimalValue({
    value: totalFiatBalance,
    fromCurrency: currentCurrency,
    conversionRate,
    invertConversionRate: true,
  });

with params:

• value: "0"
• fromCurrency: "usd"
• conversionRate: 0

Checking for valid output and returning 0x0 prevents the balance error

Manual testing steps

0. MULTICHAIN=1 yarn start
   _1. Remove your local Metamask
   _2. Re-install and go through onboarding
   _3. Get to home wallet screen without crashing

Related issues

Fixes #???

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Builds ready [4cf4d2b]

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Page Load Metrics (1234 ± 393 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	85	193	118	25	12
		domContentLoaded	70	194	111	29	14
		load	82	2057	1234	819	393
		domInteractive	70	194	111	29	14

Bundle size diffs

• background: 0 Bytes (0.00%)
• ui: 33 Bytes (0.00%)
• common: 0 Bytes (0.00%)

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (8235fa2) 68.59% compared to head (4cf4d2b) 68.59%.
Report is 7 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #21359      +/-   ##
===========================================
- Coverage    68.59%   68.59%   -0.00%     
===========================================
  Files         1026     1026              
  Lines        40979    40981       +2     
  Branches     10936    10937       +1     
===========================================
+ Hits         28108    28109       +1     
- Misses       12871    12872       +1     

Files	Coverage Δ
ui/hooks/useAccountTotalFiatBalance.js	92.31% <66.67%> (-3.53%)	⬇️

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