🥅 Return empty array for missing server response

When the server fails to send a response for certain commands, we can
workaround this by simply returning an empty array.  This should work
for `SEARCH`, `SORT`, `THREAD`, and `ENABLED`, each of which expects a
single response.

I did not do the same for `CAPABILITY` for two reasons:
* unless written carefully, it could break the cache.
* the server _must_ at least return `* CAPABILITY IMAP4rev1`, so a
  missing response may indicate a bigger problems.

lib/net/imap.rb

@@ -2188,7 +2188,7 @@ def enable(*capabilities)
         .join(' ')
       synchronize do
         send_command("ENABLE #{capabilities}")
-        result = clear_responses("ENABLED").last
+        result = clear_responses("ENABLED").last || []
         @utf8_strings ||= result.include? "UTF8=ACCEPT"
         @utf8_strings ||= result.include? "IMAP4REV2"
         result
@@ -2642,7 +2642,7 @@ def search_internal(cmd, keys, charset)
         else
           send_command(cmd, *keys)
         end
-        clear_responses("SEARCH").last
+        clear_responses("SEARCH").last || []
       end
     end
 
@@ -2691,7 +2691,7 @@ def sort_internal(cmd, sort_keys, search_keys, charset)
       normalize_searching_criteria(search_keys)
       synchronize do
         send_command(cmd, sort_keys, charset, *search_keys)
-        clear_responses("SORT").last
+        clear_responses("SORT").last || []
       end
     end
 
@@ -2704,7 +2704,7 @@ def thread_internal(cmd, algorithm, search_keys, charset)
       normalize_searching_criteria(search_keys)
       synchronize do
         send_command(cmd, algorithm, charset, *search_keys)
-        clear_responses("THREAD").last
+        clear_responses("THREAD").last || []
       end
     end
 

test/net/imap/test_imap.rb

@@ -1138,6 +1138,47 @@ def test_unselect
     end
   end
 
+  test("missing server ENABLED response") do
+    with_fake_server do |server, imap|
+      server.on "ENABLE", &:done_ok
+      enabled = imap.enable "foo", "bar", "baz"
+      assert_equal [], enabled
+    end
+  end
+
+  test("missing server SEARCH response") do
+    with_fake_server do |server, imap|
+      server.on "SEARCH",     &:done_ok
+      server.on "UID SEARCH", &:done_ok
+      found = imap.search ["subject", "hello"]
+      assert_equal [], found
+      found = imap.uid_search ["subject", "hello"]
+      assert_equal [], found
+    end
+  end
+
+  test("missing server SORT response") do
+    with_fake_server do |server, imap|
+      server.on "SORT",       &:done_ok
+      server.on "UID SORT",   &:done_ok
+      found = imap.sort ["INTERNALDATE"], ["subject", "hello"], "UTF-8"
+      assert_equal [], found
+      found = imap.uid_sort ["INTERNALDATE"], ["subject", "hello"], "UTF-8"
+      assert_equal [], found
+    end
+  end
+
+  test("missing server THREAD response") do
+    with_fake_server do |server, imap|
+      server.on "THREAD",     &:done_ok
+      server.on "UID THREAD", &:done_ok
+      found = imap.thread "REFERENCES", ["subject", "hello"], "UTF-8"
+      assert_equal [], found
+      found = imap.uid_thread "REFERENCES", ["subject", "hello"], "UTF-8"
+      assert_equal [], found
+    end
+  end
+
   private
 
   def imaps_test(timeout: 10)