[openstreetmap/openstreetmap-website] Fix map blink when active Mapnik + some overlay (PR #5570)

[Anton Khorev]

When the layers parameter `layers=` is empty (or doesn't exist), it could mean one of two things:
- keep currently enabled layers
- reset to default layers (standard base + no overlays)

osm-website's code is undecided about this:
- `map.setState` behaves as if it wants to keep the currents layers because it won't call `updateLayers` when `state.layers` are empty. `state.layers` are populated with the layers param elsewhere
- `OSM.formatHash` behaves as if empty value means standard layer because it removes `M`. Removing `M` doesn't always produce empty layers param because of overlay codes, and that's why this PR was opened.


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