bug: `pull-request-header` and `pull-request-footer` set to `""` fall back to defaults due to `||` instead of `??`

[mdrxy]

Bug

Setting "pull-request-header": "" in release-please-config.json does not remove the default header. The PR body still contains :robot: I have created a release *beep* *boop*.

Root cause

In src/util/pull-request-body.ts, the constructor uses || for both header and footer:

this.header = options?.header || DEFAULT_HEADER;
this.footer = options?.footer || DEFAULT_FOOTER;

Since empty string is falsy in JavaScript, "" || DEFAULT_HEADER evaluates to DEFAULT_HEADER. These should use nullish coalescing (??) to only fall back for null/undefined:

this.header = options?.header ?? DEFAULT_HEADER;
this.footer = options?.footer ?? DEFAULT_FOOTER;

Steps to reproduce

1. Set "pull-request-header": "" and/or "pull-request-footer": "" in release-please-config.json
2. Run release-please to create/update a release PR
3. Observe the PR body still contains the default header/footer

Expected behavior

An empty string should be respected as an intentional override, producing no header/footer.