benchmark: fix incorrect base64 input in byteLength benchmark

[semimikoh]

The benchmark for Buffer.byteLength with encoding === 'base64' does not
return a Base64-encoded string. getInput() discards the encoded value and
returns the original UTF-8 string, causing the benchmark to run against
non-base64 input.

This patch ensures that when encoding === 'base64', the function returns the
actual Base64-encoded string.

This brings the benchmark in line with its expected behavior and makes the
results meaningful.

This fixes the benchmark issue reported in #60836.
Fixes #60836.

[nodejs-github-bot]

Review requested:

• @nodejs/performance