scripts: JSON merge don't crash if no JSON found

The JSON `WORK_DIR` ($(KDIR)/json_info_files) is only created if the new
image generation methods from `image.mk` are used. However some targets
like `armvirt` do not use it yet, so the folder is never created.

The `json_overview_image_info.py` script used to raise an error if the
given `WORK_DIR` isn't a folder, however it should just notify about
missing JSON files.

This patch removes the Python assert and exists with code 0 even if no
JSON files were found, as this is not necessarily an error but simply
not yet implemented. Using `glob` on an not existing `Path` results in
an empty list, therefore the for loop won't run.

Signed-off-by: Paul Spooren <mail@aparcar.org>
CC: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit 14cbd8fb2dd8c81bad06d3c3bb45213685c19c98)

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