Module throws an error when images are served from different environment with use of stage_file_proxy

Problem/Motivation

Let's assume there are two Drupal environments: QA and PROD. Public images are stored only on PROD environment and QA is using Stage File Proxy → module to serve images directly from PROD. If ImageMagick is enabled, the following error is being thrown during some background processes, for example during search reindex:
> [error] File at 'public://path/to/file/my_image.png' does not exist (plugin: imagemagick_identify) (method: loadMetadataFromFile)

However the image itself can be displayed at the path mentioned in the error.

Steps to reproduce

1. Prepare Drupal environment "A" with modules: stage_file_proxy, imagemagick, search_api.
2. Create a node and embed some images.
3. Clone Drupal environment "A" from 1) into separe environment "B" and configure stage_file_proxy there to serve images from environment "A".
4. Try to reindex search on "B" using drush search-api-index command.

Proposed resolution

TBD

Remaining tasks

TBD

User interface changes

N/A

API changes

N/A

Data model changes

N/A