[maps] support null description in saved object (#162145)

Part of https://github.com/elastic/kibana/issues/162069

> This happens in maps too and possibly other SOs that have null
description (not sure if it were ever possible to have null description
in maps.) It seems from the SOs that Bhavya sent that it was possible
for visualizations. With this schema change the bug if fixed
https://github.com/elastic/kibana/pull/162106

> Not sure if it makes sense to also do it in maps. @nreese wdyt?

Add support for null description in maps saved object to be consistent
with visualize saved objects.

### test instructions
1) Install web logs sample data set
2) export map
3) manually change ndjson, changing `description:""` to
`description:null`
4) reimport map saved object
5) verify map can be opened

---------

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

x-pack/plugins/maps/common/content_management/v1/cm_services.ts

@@ -16,7 +16,7 @@ import {
 const mapAttributesSchema = schema.object(
   {
     title: schema.string(),
-    description: schema.maybe(schema.string()),
+    description: schema.maybe(schema.nullable(schema.string())),
     mapStateJSON: schema.maybe(schema.string()),
     layerListJSON: schema.maybe(schema.string()),
     uiStateJSON: schema.maybe(schema.string()),

x-pack/test/functional/fixtures/kbn_archiver/maps.json

@@ -77,7 +77,7 @@
 
 {
   "attributes": {
-    "description": "",
+    "description": null,
     "layerListJSON": "[{\"id\":\"n1t6f\",\"label\":null,\"minZoom\":0,\"maxZoom\":24,\"sourceDescriptor\":{\"id\":\"62eca1fc-fe42-11e8-8eb2-f2801f1b9fd1\",\"type\":\"ES_SEARCH\",\"geoField\":\"geometry\",\"limit\":2048,\"filterByMapBounds\":false,\"showTooltip\":true,\"tooltipProperties\":[\"name\"],\"applyGlobalQuery\":false,\"indexPatternRefName\":\"layer_1_source_index_pattern\",\"scalingType\":\"LIMIT\"},\"visible\":true,\"temporary\":false,\"style\":{\"type\":\"VECTOR\",\"properties\":{\"fillColor\":{\"type\":\"DYNAMIC\",\"options\":{\"fieldMetaOptions\":{\"isEnabled\":false,\"sigma\":3},\"field\":{\"label\":\"max(prop1) group by meta_for_geo_shapes*.runtime_shape_name\",\"name\":\"__kbnjoin__max_of_prop1__855ccb86-fe42-11e8-8eb2-f2801f1b9fd1\",\"origin\":\"join\"},\"color\":\"Blues\"}},\"iconSize\":{\"type\":\"STATIC\",\"options\":{\"size\":10}},\"symbolizeAs\":{\"options\":{\"value\":\"circle\"}},\"icon\":{\"type\":\"STATIC\",\"options\":{\"value\":\"marker\"}}},\"temporary\":true,\"previousStyle\":null},\"type\":\"VECTOR\",\"joins\":[{\"leftField\":\"name\",\"right\":{\"id\":\"855ccb86-fe42-11e8-8eb2-f2801f1b9fd1\",\"indexPatternTitle\":\"meta_for_geo_shapes*\",\"term\":\"runtime_shape_name\",\"metrics\":[{\"type\":\"max\",\"field\":\"prop1\"}],\"applyGlobalQuery\":true,\"indexPatternRefName\":\"layer_1_join_0_index_pattern\",\"type\":\"ES_TERM_SOURCE\"}}]}]",
     "mapStateJSON": "{\"zoom\":3.02,\"center\":{\"lon\":77.33426,\"lat\":-0.04647},\"timeFilters\":{\"from\":\"now-17m\",\"to\":\"now\",\"mode\":\"quick\"},\"refreshConfig\":{\"isPaused\":true,\"interval\":1000},\"settings\":{\"autoFitToDataBounds\":false}}",
     "title": "join example",