(pronouns) safe guard against requesting unknown morphemes

this should only happen when something is misconfigured or a wrong <GrammarTables> is supplied, but it does hurt DX oftentimes

src/classes.ts

@@ -519,7 +519,7 @@ export class Pronoun {
         }
 
         const value = this.morphemes[morpheme];
-        if (value === null) {
+        if (value === null || value === undefined) {
             return null;
         }