fix legend for one item, make layer options optional

dist/_test/compare-test.js

dist/_test/compare-test.js.map

dist/layers/LayerBase.js

dist/layers/LayerBase.js.map

dist/layers/LayerBaseVector.d.ts

dist/layers/LayerBaseVector.js

dist/layers/LayerBaseVector.js.map

dist/layers/LayerEsriMapServer.d.ts

dist/layers/LayerEsriMapServer.js

dist/layers/LayerEsriMapServer.js.map

dist/olHelpers/esriToOlStyle.js

dist/olHelpers/esriToOlStyle.js.map

src-ts/_test/compare-test.ts

src-ts/_test/simple_map.ts

src-ts/layers/LayerBase.ts

src-ts/layers/LayerBaseVector.ts

src-ts/layers/LayerEsriMapServer.ts

src-ts/olHelpers/esriToOlStyle.ts

test-html/_css/all-ol-style.less

test-html/_css/layer-swipe.less