Prevent that we unset the mail if it's not changed and stop removing the...

... field or zentyal-mail breaks as a workaround to the fact that the mail field is optional even if zentyal-mail requires it

It fixes the new added tests to the POP/IMAP test suite: (EditUser and EditUserWithEmptyMail)

	POP/IMAP tests suite
		ConfigNetworkSetExternal: OK
		EnableModules: OK
		RemoveGateway: OK
		AddGateway: OK
		AddUser: OK
		CreateVDomain: OK
		SaveChanges1: OK
		SetEmailAccount: OK
		SetupRouter: OK
		EnablePOP: OK
		Test-POP-internal: OK
		Test-POP-external: OK
		Test-NOT-POPS-internal: OK
		Test-NOT-POPS-external: OK
		Test-NOT-IMAP-internal: OK
		Test-NOT-IMAP-external: OK
		Test-NOT-IMAPS-internal: OK
		Test-NOT-IMAPS-external: OK
		EnablePOPS: OK
		Test-NOT-POP-internal: OK
		Test-NOT-POP-external: OK
		Test-POPS-internal: OK
		Test-POPS-external: OK
		Test-NOT-IMAP-internal: OK
		Test-NOT-IMAP-external: OK
		Test-NOT-IMAPS-internal: OK
		Test-NOT-IMAPS-external: OK
		EnableIMAP: OK
		Test-NOT-POP-internal: OK
		Test-NOT-POP-external: OK
		Test-NOT-POPS-internal: OK
		Test-NOT-POPS-external: OK
		Test-IMAP-internal: OK
		Test-IMAP-external: OK
		Test-NOT-IMAPS-internal: OK
		Test-NOT-IMAPS-external: OK
		EnableIMAPS: OK
		Test-NOT-POP-internal: OK
		Test-NOT-POP-external: OK
		Test-NOT-POPS-internal: OK
		Test-NOT-POPS-external: OK
		Test-NOT-IMAP-internal: OK
		Test-NOT-IMAP-external: OK
		Test-IMAPS-internal: OK
		Test-IMAPS-external: OK
                EditUser: OK
		EditUserWithEmptyMail: OK
		check-zentyal-log: OK
		check-syslog-apparmor: OK