Apply strictNullChecks to src/utils/*!exportUtils (#10455

* Apply `strictNullChecks` to `src/utils/exportUtils`

* strict fix

* fix strictNullChecks issues in some utils

* fix error message

* test coverage

* lint

* more strictNullChecks

* small optimisation for getUniqueRoomsWithIndividuals

* tidy

* test coverage

src/utils/stringOrderField.ts

@@ -120,7 +120,7 @@ export const reorderLexicographically = (
         // verify the right move would be sufficient
         if (
             rightBoundIdx === newOrder.length - 1 &&
-            (newOrder[rightBoundIdx] ? stringToBase(newOrder[rightBoundIdx].order) : BigInt(Number.MAX_VALUE)) -
+            (newOrder[rightBoundIdx]?.order ? stringToBase(newOrder[rightBoundIdx].order!) : BigInt(Number.MAX_VALUE)) -
                 prevBase <=
                 rightBoundIdx - toIndex
         ) {