Bug 313504: style: prefer QString.clear over a null-assign

style: prefer QString.clear over a null-assign

    See:
    http://www.englishbreakfastnetwork.org/krazy/reports/extragear/multimedia/amarok/index.html
    [...]
    Check for assignments to QString::null [nullstrassign]...OOPS! 6 issues found!

        src/services/amazon/AmazonStore.cpp: line# 116 (1)
        src/dialogs/TagDialog.cpp: line# 835 (1)
        src/browsers/playlistbrowser/PlaylistBrowserCategory.cpp: line# 261 (1)
        src/core-impl/collections/db/sql/SqlMeta.cpp: line# 904,1737,1886 (3)
    [...]

REVIEW: 112139
CCBUG: 313504