Fix hiding of filename label when indexer is idle

This fixes a regression caused by commit 6ea58c0f ("kcms/baloo: make
"current indexing" label a full path again"). Prior to this commit, the
condition included the boolean "monitor.currentlyIndexing" property.

Using the filePath.length and completionPercentage is insufficient,
as the filepath value is only valid when the state is *not* idle, and
may show e.g. a placeholder value during idle state.

BUG: 477274