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

## 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`](https://github.com/googleapis/release-please/blob/main/src/util/pull-request-body.ts), the constructor uses `||` for both header and footer:

```typescript
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`:

```typescript
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.