Conform more of the code base to strict null checking (#10147)

* Conform more of the code base to strict null checking

* More strict fixes

* More strict work

* Fix missing optional type

* Iterate

src/customisations/Alias.ts

@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-function getDisplayAliasForAliasSet(canonicalAlias: string, altAliases: string[]): string {
+function getDisplayAliasForAliasSet(canonicalAlias: string | null, altAliases: string[]): string | null {
     // E.g. prefer one of the aliases over another
     return null;
 }