Fix map blink when active Mapnik + some overlay

[deevroman]

#5474 added an unpleasant blink when an overlay is enabled and Mapnik is base layer, and you try to close the sidebar:

blick.mp4

This is because var layers = layerParam || "M"; does not add M when overlay is enabled (layerParam is not empty). I added a check that if Mapnik is the current layer, then we add its code, so that.removeLayer() was not called for it.

Checking this.getMapbaslaver() && is important because when opening the tab it can return undefined.

[AntonKhorev]

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.

[AntonKhorev]

But M is not removed from everywhere. The so-called _osm_location cookie which also stores currently selected layers keeps M.