Strip `None` values from symcache dict before TOML write

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

Note that (again) this patch was orig by @dnks and broken out from
a larger commit which added unnecessary/out-of-scope changes we didn't
end up requiring.

piker/data/_symcache.py

@@ -105,6 +105,15 @@ class SymbologyCache(Struct):
 
     def write_config(self) -> None:
 
+        def clean_dict_for_toml(d):
+            '''Remove None values from dict recursively for TOML serialization'''
+            if isinstance(d, dict):
+                return {k: clean_dict_for_toml(v) for k, v in d.items() if v is not None}
+            elif isinstance(d, list):
+                return [clean_dict_for_toml(item) for item in d if item is not None]
+            else:
+                return d
+
         # put the backend's pair-struct type ref at the top
         # of file if possible.
         cachedict: dict[str, Any] = {
@@ -125,7 +134,9 @@ class SymbologyCache(Struct):
 
             dct = cachedict[key] = {}
             for key, struct in table.items():
-                dct[key] = struct.to_dict(include_non_members=False)
+                raw_dict = struct.to_dict(include_non_members=False)
+                # Clean None values for TOML compatibility
+                dct[key] = clean_dict_for_toml(raw_dict)
 
         try:
             with self.fp.open(mode='wb') as fp: