Powerpoint writer: avoid extra blank lines before author.

src/Text/Pandoc/Writers/Powerpoint/Output.hs

test/pptx/document-properties-short-desc/output.pptx

test/pptx/document-properties-short-desc/templated.pptx

test/pptx/footer/basic/output.pptx

test/pptx/footer/fixed-date/output.pptx

test/pptx/footer/higher-slide-number/output.pptx

test/pptx/footer/no-title-slide/output.pptx

test/pptx/layouts/deleted.pptx

test/pptx/layouts/moved.pptx

test/pptx/speaker-notes-after-metadata/output.pptx

test/pptx/speaker-notes-after-metadata/templated.pptx