no mail are sent if categories not in use

[mxsteini]

if categories are not in use, the function Dmail->dmailer_getBoundaryParts set $this->mailHasContent to false. This is because den $userCategories contains an empty string. In line 395 this string is checked by intval which return 0 in this case not -1. Because intval never return -1, this test ever failes.