Fix: Prevent redundant OUT column creation and add null check

- Moved OUT column creation outside inner loop to avoid redundant operations
- Added null check for pdb_records[r] to prevent processing undefined records
- Combined performance optimization with robustness improvement

Author: fazhang-master

biopandas/pdb/pandas_pdb.py

@@ -715,9 +715,10 @@ def to_pdb(self, path, records=None, gz=False, append_newline=True):
         dfs = {r: self.df[r].copy() for r in records if not self.df[r].empty}
 
         for r in dfs:
-            for col in pdb_records[r]:
-                dfs[r][col["id"]] = dfs[r][col["id"]].apply(col["strf"])
+            if pdb_records[r]:
                 dfs[r]["OUT"] = pd.Series("", index=dfs[r].index)
+                for col in pdb_records[r]:
+                    dfs[r][col["id"]] = dfs[r][col["id"]].apply(col["strf"])
 
             for c in dfs[r].columns:
                 # fix issue where coordinates with four or more digits would
@@ -946,9 +947,10 @@ def to_pdb_stream(self, records: tuple[str] = ("ATOM", "HETATM")) -> StringIO:
         dfs = {r: df.loc[df.record_name == r] for r in records}
 
         for r in dfs:
-            for col in pdb_records[r]:
-                dfs[r][col["id"]] = dfs[r][col["id"]].apply(col["strf"])
+            if pdb_records[r]:
                 dfs[r]["OUT"] = pd.Series("", index=dfs[r].index)
+                for col in pdb_records[r]:
+                    dfs[r][col["id"]] = dfs[r][col["id"]].apply(col["strf"])
 
             for c in dfs[r].columns:
                 # fix issue where coordinates with four or more digits would