DAP: accept requests with '"arguments": null'

Some Debug Adapter Protocol clients like Helix set the optional
"arguments" field of the ConfigurationDone request to null, which is a
bit odd but seems to be allowed by the protocol specification. Before
this patch, Python exceptions would be raised on such requests. This
patch makes it so these requests are treated as if the "arguments"
field was absent.

gdb/python/lib/gdb/dap/server.py

@@ -312,7 +312,15 @@ class Server:
         }
 
         if "arguments" in params:
-            args = params["arguments"]
+            # Since the "arguments" field is optional, setting it to
+            # null is an odd thing to do when one could simply omit it
+            # entirely. But some clients do just that for some
+            # requests (e.g. Helix for ConfigurationDone), so we
+            # accommodate this case.
+            if params["arguments"] is None:
+                args = {}
+            else:
+                args = params["arguments"]
         else:
             args = {}