Allow empty strings for both exclude and include (#159)

Fixes #

Description of change:

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custom_components/ics_calendar/config_flow.py

@@ -148,7 +148,10 @@ async def async_step_calendar_opts(
         """Calendar Options step for ConfigFlow."""
         errors = {}
         if user_input is not None:
-            if user_input[CONF_EXCLUDE] == user_input[CONF_INCLUDE]:
+            if (
+                user_input[CONF_EXCLUDE]
+                and user_input[CONF_EXCLUDE] == user_input[CONF_INCLUDE]
+            ):
                 errors[CONF_EXCLUDE] = "exclude_include_cannot_be_the_same"
             if user_input[CONF_DOWNLOAD_INTERVAL] < 15:
                 _LOGGER.error("download_interval_too_small error")