fw4: gracefully handle `null` return values from `fd.read("line")`

fw4: gracefully handle `null` return values from `fd.read("line")`

Since ucode commit 4ae7072 "fs: use `getline()` for line wise read operations"
a call to `fs.read("line")` will yield `null` instead of an empty string when
encountering EOF, leading to an infinite loop when parsing loadfile entries.

Solve this issue by checking both for `null` and empty string return values.

Ref: https://forum.openwrt.org/t/x/139951/12
Signed-off-by: Jo-Philipp Wich <jo@mein.io>