In interactive mode, when it prompts for a password, just press enter and there will be no password set.
If you are want to automate that (for example as an ansible command), use the -passout argument. It expects the parameter to be in the form pass:mypassword. Since we want no password:
openssl pkcs12 -export -nodes -out bundle.pfx -inkey mykey.key \
-in certificate.crt -certfile ca-cert.crt \
-passout pass: