boxes/views: Treat spaces as an empty search in PanelSearchBox.

This amends PanelSearchBox and related update functions to treat spaces
as an empty search by using left strip, i.e., the search is
canceled and the list is not updated.

Author: preetmishra

zulipterminal/ui_tools/boxes.py

@@ -1213,7 +1213,8 @@ def reset_search_text(self) -> None:
         self.set_edit_text(self.search_text)
 
     def keypress(self, size: urwid_Size, key: str) -> Optional[str]:
-        if ((is_command_key('ENTER', key) and self.get_edit_text() == '')
+        if ((is_command_key('ENTER', key)
+             and self.get_edit_text().lstrip() == '')
                 or is_command_key('GO_BACK', key)):
             self.panel_view.view.controller.exit_editor_mode()
             self.reset_search_text()

zulipterminal/ui_tools/views.py

@@ -283,6 +283,8 @@ def __init__(self, streams_btn_list: List[Any], view: Any) -> None:
 
     @asynch
     def update_streams(self, search_box: Any, new_text: str) -> None:
+        new_text = new_text.lstrip()
+
         if not self.view.controller.is_in_editor_mode():
             return
         # wait for any previously started search to finish to avoid
@@ -369,6 +371,8 @@ def __init__(self, topics_btn_list: List[Any], view: Any,
 
     @asynch
     def update_topics(self, search_box: Any, new_text: str) -> None:
+        new_text = new_text.lstrip()
+
         if not self.view.controller.is_in_editor_mode():
             return
         # wait for any previously started search to finish to avoid
@@ -626,6 +630,9 @@ def update_user_list(self, search_box: Any=None,
         if not self.allow_update_user_list and new_text is None:
             return
 
+        if new_text is not None:
+            new_text = new_text.lstrip()
+
         # wait for any previously started search to finish to avoid
         # displaying wrong user list.
         with self.search_lock: