improvement(swap): hide swap placeholders to reduce visual complexity (#1678)

* resolve #1646: hide swap placeholders to reduce visual complexity

* use empty string over space

src/components/CurrencyInputPanel/FiatValue.tsx

@@ -24,8 +24,8 @@ export function FiatValue({
 
   return (
     <TYPE.body fontSize={14} color={fiatValue ? theme.text2 : theme.text4}>
-      {fiatValue ? '~' : ''}$
-      <HoverInlineText text={fiatValue ? fiatValue?.toSignificant(6, { groupSeparator: ',' }) : '-'} />{' '}
+      {fiatValue ? '~$' : ''}
+      <HoverInlineText text={fiatValue ? fiatValue?.toSignificant(6, { groupSeparator: ',' }) : ''} />{' '}
       {priceImpact ? (
         <span style={{ color: priceImpactColor }}> ({priceImpact.multiply(-1).toSignificant(3)}%)</span>
       ) : null}

src/components/CurrencyInputPanel/index.tsx

@@ -273,14 +273,14 @@ export default function CurrencyInputPanel({
                         formatTokenAmount(selectedCurrencyBalance, 4) +
                         ' ' +
                         currency.symbol
-                      : '-'}
+                      : ''}
                   </TYPE.body>
                   {showMaxButton && selectedCurrencyBalance ? (
                     <StyledBalanceMax onClick={onMax}>(Max)</StyledBalanceMax>
                   ) : null}
                 </RowFixed>
               ) : (
-                '-'
+                <span></span>
               )}
               <FiatValue fiatValue={fiatValue} priceImpact={priceImpact} />
             </RowBetween>