Uncaught type error when signingKey is missing a key property

[KalleV]

Description

The library throws an uncaught exception instead of passing an error to the callback when the signingKey "key" property is missing.

Reproduction

Configure the strategy with the "signingKey" property and specify the "cert" but not the "key". For example:

{
  signingKey: {
    cert: 'awebrewobuwebao'
  }
}