Fix saving email notification settings with an empty From address

[Marijn0]

The backend input_sanity_check() throws a error:

raise ValueError(
ERROR: Value "" for setting "email_notifications_from" did not match regex "^[A-Za-z0-9 _+.@^~<>,-]+$": Email addresses are only allowed to contain alphanumerical characters, underscore _, plus +, dot ., at @, caret ^, tilde ~, angle brackets <>, hyphen -, and may be separated by comma, and space

when the From address field in the email notification settings is empty, preventing the settings from being saved. The sanity-check regex requires at least one character. The frontend does not require this field to be set. The UI help mark says:

motioneye/motioneye/locale/en/LC_MESSAGES/motioneye.po

Line 1358 in da7e73b

msgstr "set a custom From address, if your SMTP service requires one (the first destination email address will be used if left blank)."

We now skip the sanity check when email_notifications_from is empty. I guess we could also edit the regex itself if preferred, but it is shared across multiple input fields.