Fix no-eq-null issues (#9205)

See [`no-eq-null`](https://eslint.org/docs/rules/no-eq-null) for more information.

This change enables `no-eq-null` and fixes the issues raised by the rule.

.eslintrc.js

@@ -49,6 +49,7 @@ module.exports = {
     'guard-for-in': 'error',
     'no-case-declarations': 'error',
     'no-empty': 'error',
+    'no-eq-null': 'error',
     'no-loop-func': 'error',
     'no-useless-catch': 'error',
     'no-useless-concat': 'error',

ui/app/components/app/asset-list-item/asset-list-item.js

@@ -60,7 +60,7 @@ const AssetListItem = ({
     : null
 
   const sendTokenButton = useMemo(() => {
-    if (tokenAddress == null) {
+    if (tokenAddress === null || tokenAddress === undefined) {
       return null
     }
     return (

ui/app/helpers/utils/conversion-util.js

@@ -99,7 +99,7 @@ const converter = ({
   }
 
   if (fromCurrency !== toCurrency) {
-    if (conversionRate == null) {
+    if (conversionRate === null || conversionRate === undefined) {
       throw new Error(`Converting from ${fromCurrency} to ${toCurrency} requires a conversionRate, but one was not provided`)
     }
     let rate = toBigNumber.dec(conversionRate)

ui/app/helpers/utils/i18n-helper.js

@@ -56,7 +56,10 @@ export const getMessage = (localeCode, localeMessages, key, substitutions) => {
         return part
       }
       const substituteIndex = Number(subMatch[1]) - 1
-      if (substitutions[substituteIndex] == null && !missingSubstitutionErrors[localeCode]?.[key]) {
+      if (
+        (substitutions[substituteIndex] === null || substitutions[substituteIndex] === undefined) &&
+        !missingSubstitutionErrors[localeCode]?.[key]
+      ) {
         if (!missingSubstitutionErrors[localeCode]) {
           missingSubstitutionErrors[localeCode] = {}
         }

ui/app/hooks/useTransactionDisplayData.js

@@ -99,7 +99,7 @@ export function useTransactionDisplayData (transactionGroup) {
   // 3. Deposit
   // 4. Site interaction
   // 5. Approval
-  if (transactionCategory == null) {
+  if (transactionCategory === null || transactionCategory === undefined) {
     category = TRANSACTION_CATEGORY_SIGNATURE_REQUEST
     title = t('signatureRequest')
     subtitle = origin