remove empty board keywords

ci/release/changelogs/next.md

@@ -7,6 +7,7 @@
 - Lib: removes a dependency on external slog that was causing troubles with installation [#2137](https://github.com/terrastruct/d2/pull/2137)
 - CLI: attempts writing to path atomically, falling back to non-atomic if failed [#2141](https://github.com/terrastruct/d2/pull/2141)
 - Export: pptx has "created at" metadata removed, so successive runs yield the same result [#2169](https://github.com/terrastruct/d2/pull/2160)
+- Formatter: empty board keywords (e.g. `layers`) are removed [#2178](https://github.com/terrastruct/d2/pull/2178)
 
 #### Bugfixes ⛑️
 

d2format/format.go

@@ -293,12 +293,19 @@ func (p *printer) _map(m *d2ast.Map) {
 		if nb.IsBoardNode() {
 			switch nb.MapKey.Key.Path[0].Unbox().ScalarString() {
 			case "layers":
+				// remove useless
+				if nb.MapKey.Value.Map != nil && len(nb.MapKey.Value.Map.Nodes) > 0 {
 					layerNodes = append(layerNodes, nb)
+				}
 			case "scenarios":
+				if nb.MapKey.Value.Map != nil && len(nb.MapKey.Value.Map.Nodes) > 0 {
 					scenarioNodes = append(scenarioNodes, nb)
+				}
 			case "steps":
+				if nb.MapKey.Value.Map != nil && len(nb.MapKey.Value.Map.Nodes) > 0 {
 					stepNodes = append(stepNodes, nb)
 				}
+			}
 			prev = n
 			continue
 		}

d2format/format_test.go

@@ -881,6 +881,17 @@ coop: {
     fill: blue
   }
 }
+`,
+		},
+		{
+			name: "remove-empty-boards",
+			in: `k
+
+layers
+scenarios: {}
+steps: asdf
+`,
+			exp: `k
 `,
 		},
 	}