Minor cleanup: php-cs-fixer, tests, interface consistency

IMipPlugin.php Removed blank lines to make php-cs-fixer happy.
Minor cleanup:  bugs found by Psalm static checker
IEMailTemplate: The public interface to addBodyListItem also needs to include the new plainIndent parameter.
IMipPlugin: Fixes an undefined variable for events that do not have DTEND.  Also use explicit string conversion for parameters and properties in several places.

The new email template adds an additional blank line before "button" links in plain text, so the tests were fixed to include that additional blank line.

Signed-off-by: Brad Rubenstein <brad@wbr.tech>