App.get_values(): Filter out empty split results

get_values() splits comma-separated values and strips whitespace but does not filter out empty strings. A value like "a, b, " produces ['a', 'b', ''], with an empty string at the end. This empty string propagates to callers like CmdRequiredOsPkg.py and pollutes output. Add a filter for non-empty stripped values.
Assisted-by: unsloth/Qwen3.6-35B-A3B-GGUF:IQ4_NL and pi.dev
Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/App.py

@@ -584,7 +584,10 @@ class App(Base):
                 for key in keys:
                     vals = self.get_value(p, section, key)
                     if vals:
-                        ret += [val.strip() for val in vals.split(',')]
+                        for val in vals.split(','):
+                            stripped = val.strip()
+                            if stripped:
+                                ret.append(stripped)
         return list(dict.fromkeys(ret))  # Remove duplicates, keep ordering
 
     def get_project_refs(