[second tree control] show only when available

Don't display the sidebar UI if we're just going to display an empty
dropdown!

See <https://bedfordlab.slack.com/archives/C0K3GS3J8/p1719424903252149>
for context

CHANGELOG.md

@@ -1,5 +1,8 @@
 # Changelog
 
+* We no longer show the "second tree" sidebar dropdown when there are no available options. The possible options are defined by [the charon/getAvailable API](https://docs.nextstrain.org/projects/auspice/en/stable/server/api.html) response and as such vary depending on the server in use. ([#1795](https://github.com/nextstrain/auspice/pull/1795))
+
+
 ## version 2.55.1 - 2024/06/25
 
 

src/components/controls/choose-second-tree.js

@@ -33,6 +33,9 @@ class ChooseSecondTree extends React.Component {
         .filter((opt) => !!opt) // .secondTreeOptions is not required
     )]
 
+    // Don't display the sidebar UI if we're just going to display an empty dropdown!
+    if (!options.length) return null;
+
     if (this.props.showTreeToo) options.unshift("REMOVE");
 
     const selectOptions = options.map((opt) => ({value: opt, label: opt}));