bug(ios-rust): fix bad substitution in ios build script

[relud]

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[relud]

this is valid in the default version of bash installed on macos, but only because it's silently ignoring the :-0 part. in later versions of bash, it fails with a bad substitution error.

Also, this conditional is unnecessary, because "${GENERATE_ARGS[@]}" already does the right thing here. tested on bash 3.2.57 (mac os 26.2 default) and bash 5.3.9 (installed by homebrew).

[skhamis]

Thanks for fixing this! Looks good!