Fix return value of PageView::capabilityFlags

The return type of that method is Okular::View::CapabilityFlags.
Hence, returning 'nullptr' as was done previously cannot be correct,
and 'NoFlag' should be used instead.  As both evaluate to '0'
internally there should not be any functional difference.
2 jobs for !39 with save-view-modes in 20 minutes and 52 seconds (queued for 14 seconds)
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Status Job ID Name Coverage
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passed #17121
build_ubuntu_18_04

00:08:21

passed #17120
linux kf5-qt5 qt5.12

00:13:55

41.0%