Do not set initial empty search query

The way that the search interface is currently implemented,
clicking on the "search" icon in the top bar actually
submits the search form (with an empty query).

Ignoring empty queries if a query has not yet been set
alleviates "searching" for nothing.

This also prevents expanded threads from being re-collapsed
when clicking on the search icon.

src/sidebar/components/search-input.js

@@ -29,7 +29,11 @@ export default function SearchInput({ alwaysExpanded, query, onSearch }) {
 
   const onSubmit = e => {
     e.preventDefault();
+    if (input.current.value || prevQuery) {
+      // Don't set an initial empty query, but allow a later empty query to
+      // clear `prevQuery`
       onSearch(input.current.value);
+    }
   };
 
   // When the active query changes outside of this component, update the input

src/sidebar/components/test/search-input-test.js

@@ -61,6 +61,25 @@ describe('SearchInput', () => {
     assert.calledWith(onSearch, 'new-query');
   });
 
+  it('does not set an initial empty query when form is submitted', () => {
+    // If the first query entered is empty, it will be ignored
+    const onSearch = sinon.stub();
+    const wrapper = createSearchInput({ onSearch });
+    typeQuery(wrapper, '');
+    wrapper.find('form').simulate('submit');
+    assert.notCalled(onSearch);
+  });
+
+  it('sets subsequent empty queries if entered', () => {
+    // If there has already been at least one query set, subsequent
+    // empty queries will be honored
+    const onSearch = sinon.stub();
+    const wrapper = createSearchInput({ query: 'foo', onSearch });
+    typeQuery(wrapper, '');
+    wrapper.find('form').simulate('submit');
+    assert.calledWith(onSearch, '');
+  });
+
   it('renders loading indicator when app is in a "loading" state', () => {
     fakeStore.isLoading.returns(true);
     const wrapper = createSearchInput();