CSVExportFilter segfaults if no filename is specified

[timp3289]

Windows 11, msys64, recent build from source.

• Export file name is blank by default
• CSVExportFilter does not check for non-blank name before opening the file
• CSVExportFilter does not check for non-null FILE* before trying to fprintf()
• Segfault on line 187 of CSVExportFilter.cpp