Fix: to avoid re-parsing SigmaString entries in values

Optimized value conversion to SigmaString by avoiding re-parsing existing SigmaString entries

Author: vl43den

sigma/processing/transformations/values.py

@@ -380,10 +380,16 @@ def apply_value(
         if self.target_type == "str":
             if isinstance(val, SigmaExpansion):
                 for i, entry in enumerate(val.values):
-                    val.values[i] = SigmaString(str(entry))
+                    # avoid re-parsing entries that are already SigmaString
+                    if not isinstance(entry, SigmaString):
+                        val.values[i] = SigmaString(str(entry))
 
                 return val
 
+            # confirming correct structure, avoiding re-parsing
+            if isinstance(val, SigmaString):
+                return val
+
             return SigmaString(str(val))
         elif self.target_type == "num":
             try: