Fix mail transporter verification

[SimonRosenau]

In case of json transporters, or any transporter which does not implement the verify function, the Mailer class inside nodemailer overwrites it with a generic non-async function, which just returns false. This leads to the following error:

2024-04-03 23:25:19 TypeError: transporter.verify(...).then is not a function
2024-04-03 23:25:19     at MailerService.verifyTransporter (/app/node_modules/@nestjs-modules/mailer/dist/mailer.service.js:67:14)
2024-04-03 23:25:19     at new MailerService (/app/node_modules/@nestjs-modules/mailer/dist/mailer.service.js:60:18)
2024-04-03 23:25:19     at Injector.instantiateClass (/app/node_modules/@nestjs/core/injector/injector.js:365:19)
2024-04-03 23:25:19     at callback (/app/node_modules/@nestjs/core/injector/injector.js:65:45)
2024-04-03 23:25:19     at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
2024-04-03 23:25:19     at async Injector.resolveConstructorParams (/app/node_modules/@nestjs/core/injector/injector.js:144:24)
2024-04-03 23:25:19     at async Injector.loadInstance (/app/node_modules/@nestjs/core/injector/injector.js:70:13)
2024-04-03 23:25:19     at async Injector.loadProvider (/app/node_modules/@nestjs/core/injector/injector.js:97:9)
2024-04-03 23:25:19     at async /app/node_modules/@nestjs/core/injector/instance-loader.js:56:13
2024-04-03 23:25:19     at async Promise.all (index 4)

By wrapping the call to transporter.verify() inside Promise.resolve we can safely proceed with the existing flow.

See nodemailer source here: https://github.com/nodemailer/nodemailer/blob/fa63b52d8a03da49a60c00b6c073a0d77aeababb/lib/mailer/index.js#L95-L115