fix: map existence check: use undefined compare

exact compare with undefined instead of truthy/null check is 12% faster
https://jsbench.me/q9kja8u4c2/2

lib/mixins/decoration-management.js

@@ -189,7 +189,7 @@ export default class DecorationManagement {
       const marker = markers[i]
       const decorations = this.decorationsByMarkerId.get(marker.id)
 
-      if (decorations != null) {
+      if (decorations !== undefined) {
         decorationsByMarkerId[marker.id] = decorations
       }
     }
@@ -353,7 +353,7 @@ export default class DecorationManagement {
 
         this.invalidateDecorationForScreenRowsCache()
 
-        if (decorations != null) {
+        if (decorations !== undefined) {
           for (let i = 0, len = decorations.length; i < len; i++) {
             const decoration = decorations[i]
             this.emitter.emit('did-change-decoration', {
@@ -527,16 +527,16 @@ export default class DecorationManagement {
     this.decorationsById.delete(decoration.id)
 
     subscription = this.decorationUpdatedSubscriptions.get(decoration.id)
-    if (subscription != null) { subscription.dispose() }
+    if (subscription !== undefined) { subscription.dispose() }
 
     subscription = this.decorationDestroyedSubscriptions.get(decoration.id)
-    if (subscription != null) { subscription.dispose() }
+    if (subscription !== undefined) { subscription.dispose() }
 
     this.decorationUpdatedSubscriptions.delete(decoration.id)
     this.decorationDestroyedSubscriptions.delete(decoration.id)
 
     const decorations = this.decorationsByMarkerId.get(marker.id)
-    if (!decorations) { return }
+    if (decorations === undefined) { return }
 
     this.emitDecorationChanges(decoration.getProperties().type, decoration)
 
@@ -566,7 +566,7 @@ export default class DecorationManagement {
     if (marker == null) { return }
 
     const decorations = this.decorationsByMarkerId.get(marker.id)
-    if (!decorations) { return }
+    if (decorations === undefined) { return }
 
     for (let i = 0, len = decorations.length; i < len; i++) {
       const decoration = decorations[i]