Add safe handling of Excel error values with ignore_invalid_char_data option

- Add ignore_invalid_char_data option to suppress errors from Excel error values like #N/A, #VALUE!, etc.
- Fix ValueError/OverflowError handling to use empty string instead of None to prevent runtime crashes
- Addresses the core issue from PR #244 with a safer implementation that avoids string concatenation errors
- All existing tests pass, maintaining backward compatibility

Resolves the problem described in #244

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: dilshod

xlsx2csv.py

@@ -219,6 +219,7 @@ def __init__(self, xlsxfile, **options):
         options.setdefault("lineterminator", "\n")
         options.setdefault("outputencoding", "utf-8")
         options.setdefault("skip_hidden_rows", True)
+        options.setdefault("ignore_invalid_char_data", False)
 
         self.options = options
         self.py3 = sys.version_info[0] == 3
@@ -966,7 +967,12 @@ def handleCharData(self, data):
                             self.data = ("%f" % data).rstrip('0').rstrip('.')
 
                 except (ValueError, OverflowError):  # this catch must be removed, it's hiding potential problems
-                    raise XlsxValueError("Error: potential invalid date format.")
+                    if self.options['ignore_invalid_char_data']:
+                        # If invalid character data or excel formulas are encountered,
+                        # we set the data to empty string to avoid conversion errors
+                        self.data = ""
+                    else:
+                        raise XlsxValueError("Error: potential invalid date format.")
 
     def handleStartElement(self, name, attrs):
         has_namespace = name.find(":") > 0