Fix warnings when `filter_the_content` cannot find a valid attachment id

This code in [inc/class-tachyon.php](https://github.com/humanmade/tachyon-plugin/blob/e41d716e7455f8f7a07cf2631bf93c5b58932176/inc/class-tachyon.php#L322). Can cause warnings.

```php
$src_per_wp = wp_get_attachment_image_src( $attachment_id, $size );

if ( self::validate_image_url( $src_per_wp[0] ) ) {
$src = $src_per_wp[0];
$fullsize_url = true;
...
```

If the return from `wp_get_attachment_image_src` is `false`, which it can be, the code should not continue as if it got the array of image data.

The call to `validate_image_url` generates a warning:
"Trying to access array offset on value of type bool"

and then the first line in `validate_image_url` generates another warning:
"parse_url(): Passing null parameter #1 ($url) of type string is deprecated"

I don't understand how the $attachement_id might be invalid, or reference an invalid image, but we know this does happen in the wild.

---

## Acceptance Criteria

- [ ] Content with invalid attachment id does not generate these warnings
- [ ] Document testing on the ticket

---

## For Altis Team Use

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