Problem with Mohr Siebeck citation

Hi all,
I have the following problem: Style change citation Mohr Siebeck - Law fits very well with one exception. When I save archival material as "magazine article" or "document" for example, a comma always appears in front of the title in the footnote even though there is no author.
For example:

, UAT 123/456 Postcard Alps

but it should read

UAT 123/456 Postcard Alps

Can I somehow achieve that in "Documents" the citation of the author is waived and therefore also the comma disappears. The function "Suppress author" has unfortunately brought no relief.

Many thanks in advance

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