DefaultKeyMaterialService doesn't check for null values being returned from registered ISigningCredentialStore's

[jhbritton-RSK]

Steps to Reproduce:

1. Using an existing Duende database where the keys table is empty
2. Connect an instance of Open.IdentityServer to this database
3. Register the compatibility key store using the extension .AddCompatibilityKeyStores()
4. Load discovery document in browser

Exception will be thrown with unexpected null value.