Windows MDM identity certs missing check (#12702)

This is related #12701 

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

- [X] Changes file added for user-visible changes in `changes/` or
`orbit/changes/`.
- [X] Manual QA for all new/changed functionality

changes/bug-12701-windows-mdm-missing-identity-certs

@@ -0,0 +1 @@
+* Fleet panicked when Windows MDM identity providers were not set and MDM programmatic enrollment was performed.

server/mdm/microsoft/wstep.go

@@ -88,10 +88,18 @@ func newManager(store CertStore, certPEM []byte, privKeyPEM []byte) (*manager, e
 }
 
 func (m *manager) IdentityFingerprint() string {
+	if m == nil {
+		return ""
+	}
+
 	return m.identityFingerprint
 }
 
 func (m *manager) IdentityCert() x509.Certificate {
+	if m == nil {
+		return x509.Certificate{}
+	}
+
 	return *m.identityCert
 }
 
@@ -99,6 +107,10 @@ func (m *manager) IdentityCert() x509.Certificate {
 // subject is the DeviceID of the about to be MDM enrolled device, it will be used as the CommonName of the certificate
 // clientCSR is the client certificate signing request
 func (m *manager) SignClientCSR(ctx context.Context, subject string, clientCSR *x509.CertificateRequest) ([]byte, string, error) {
+	if m == nil {
+		return nil, "", errors.New("windows mdm identity keypair was not configured")
+	}
+
 	if m.identityCert == nil || m.identityPrivateKey == nil {
 		return nil, "", errors.New("invalid identity certificate or private key")
 	}

server/service/microsoft_mdm.go

@@ -1045,6 +1045,10 @@ func (svc *Service) GetAuthorizedSoapFault(ctx context.Context, eType string, or
 }
 
 func (svc *Service) SignMDMMicrosoftClientCSR(ctx context.Context, subject string, csr *x509.CertificateRequest) ([]byte, string, error) {
+	if svc.wstepCertManager == nil {
+		return nil, "", errors.New("windows mdm identity keypair was not configured")
+	}
+
 	cert, fpHex, err := svc.wstepCertManager.SignClientCSR(ctx, subject, csr)
 	if err != nil {
 		return nil, "signing wstep client csr", ctxerr.Wrap(ctx, err)