fix: Return 0 as number flag value instead of undefined

[rolodato]

Couldn't figure out how to add a test case to https://github.com/Flagsmith/engine-test-data/blob/e43097ee273fa63dbb9589c02f89df3d10b51044/data/environment_n9fbf9h3v4fFgH3U3ngWhb.json - verified manually with the following test:

// test.cjs

const Flagsmith = require('./build/cjs').Flagsmith

const flagsmith = new Flagsmith({
    environmentKey: 'LMVKS7tqAcQrjtsDWLoe7g',
});

const flags = await flagsmith.getEnvironmentFlags();
const value = flags.getFeatureValue('zero');

console.log(value);

Before:

rolodato@rolosmith flagsmith-nodejs-client % bun run build && bun test.cjs
$ rm -rf ./build
$ tsc -b tsconfig.cjs.json tsconfig.esm.json && echo '{"type": "commonjs"}'> build/cjs/package.json
undefined

After:

rolodato@rolosmith flagsmith-nodejs-client % bun run build && bun test.cjs 
$ rm -rf ./build
$ tsc -b tsconfig.cjs.json tsconfig.esm.json && echo '{"type": "commonjs"}'> build/cjs/package.json
0