Always return a Boolean from `getShowFiatSelector` (#8585)

`getShowFiatSelector` would return the `showFiatInTestnets` value
directly when on mainnet, which could be `undefined`. Now it is cast to
a Boolean instead.

This was done to fix a PropType warning in a component that will be
included in a future PR, where this selector was used for a required
prop.

ui/app/selectors/selectors.js

@@ -288,7 +288,7 @@ export function preferencesSelector ({ metamask }) {
 export function getShouldShowFiat (state) {
   const isMainNet = getIsMainnet(state)
   const { showFiatInTestnets } = preferencesSelector(state)
-  return isMainNet || showFiatInTestnets
+  return Boolean(isMainNet || showFiatInTestnets)
 }
 
 export function getAdvancedInlineGasShown (state) {