Add filter to ensure publicKey is defined

Author: pallakartheekreddy

src/services/certificate/implementation/certificate-service-impl.ts

@@ -1,5 +1,5 @@
 import { Container, inject, injectable } from "inversify";
-import { catchError, map, mergeMap } from "rxjs/operators";
+import { catchError, filter, map, mergeMap } from "rxjs/operators";
 import { CsLearnerCertificate, CsLearnerCertificateV1, CsLearnerCertificateV2 } from "../../../models";
 import { CSGetLearnerCerificateRequest } from "..";
 import { defer, iif, Observable, of } from "rxjs";
@@ -347,6 +347,7 @@ export class CertificateServiceImpl implements CsCertificateService {
                         })
                     )
                     .pipe(
+                        filter((publicKey): publicKey is string => !!publicKey),
                         mergeMap((publicKey) => {
                             return new CertificateVerifier(this.httpService, this.container).verifyData(JSON.parse(response._osSignedData), publicKey).then((data) => {
                                 return  {