base-files: fix offset conversion on caldata_patch_data()

base-files: fix offset conversion on caldata_patch_data()

`$(( ))` will convert uninitialized variable to "0". If we want to
use "-n" to check the string length, it's necessary to make sure the
converted variable is not empty.

Fixes: 652a6677d5fa ("base-files: Add new functions for ath11k caldata")
Fixes: https://github.com/openwrt/openwrt/issues/17818
Signed-off-by: Shiji Yang <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/17892
Signed-off-by: Hauke Mehrtens <[email protected]>