fix email test without "en" i18n

[Rileran]

When generating a project with i18n support, but without choosing English as either the default language or an additional language, a test case in MailServiceIT won't pass as the language key "en" is hardcoded.

I have included a log file showing the result of mvnw -ntp -Pprod verify with the given configuration. This is the interesting part

[INFO] Running com.mycompany.myapp.service.MailServiceIT
2022-03-28 11:48:10.804  WARN 2694 --- [           main] com.mycompany.myapp.service.MailService  : Email could not be sent to user 'john.doe@example.com'

org.springframework.mail.MailSendException: null

[ERROR] Tests run: 10, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 0.758 s <<< FAILURE! - in com.mycompany.myapp.service.MailServiceIT
[ERROR] com.mycompany.myapp.service.MailServiceIT.testSendEmailFromTemplate  Time elapsed: 0.009 s  <<< ERROR!
org.springframework.context.NoSuchMessageException: No message found under code 'email.test.title' for locale 'en'.
	at com.mycompany.myapp.service.MailServiceIT.testSendEmailFromTemplate(MailServiceIT.java:140)```

This was tested using docker image jhipster/jhipster:v7.7.0.

verify.log

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[CLAassistant]

All committers have signed the CLA.

[DanielFran]

Thanks @Rileran for the contribution