I have two FRITZ!Box 7430 devices (identical except for that one of them was shipped more recently) and can confirm that this issue is present on one of them. For the other, the firmware extraction appears to succeed with the default options.
I've attached a boot log showing how the kernel fails to configure the ath9k device with the empty firmware file on OpenWrt 21.02.0.
The fix suggested above succeeds in allowing the system to recognize the device, however it is not configured correctly. The MAC address (and published BSSID) seems to be stuck to "00:02:03:04:05:08", although fritz_tffs_nand can extract the correct MAC address from /dev/mtd1.