Fix duration in cli.create (#86)

Author: Marenz

src/frequenz/client/dispatch/__main__.py

@@ -58,6 +58,8 @@ async def cli(ctx: click.Context, url: str, key: str) -> None:
     if ctx.obj is None:
         ctx.obj = {}
 
+    click.echo(f"Using API URL: {url}", err=True)
+
     ctx.obj["client"] = Client(
         server_url=url,
         key=key,
@@ -264,6 +266,10 @@ async def create(
     # Remove keys with `None` value
     kwargs = {k: v for k, v in kwargs.items() if v is not None}
 
+    # Required for client.create
+    if not kwargs.get("duration"):
+        kwargs["duration"] = None
+
     dispatch = await ctx.obj["client"].create(
         recurrence=parse_recurrence(kwargs),
         **kwargs,