fix: remove -u from build_wasm script so that we can skip the build when SKIP_CPP_BUILD is unset

[kevaundray]

The build wasm script uses set -u so if SKIP_CPP_BUILD is not set, then the script will error

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[kevaundray]

Actually -z will check if the string is empty or unset, so I think the -u was not meant to be there.

For example, the following will not skip the build:

SKIP_CPP_BUILD=0

if [ -z "$SKIP_CPP_BUILD" ]; then
    echo "SKIP_CPP_BUILD is unset or empty"
else
    echo "SKIP_CPP_BUILD is set and not empty"
fi

whereas

SKIP_CPP_BUILD="" will skip the build. Not setting SKIP_CPP_BUILD will result in a failure due to set -u