[3.0] Stop the package browser dying without a cached version list

[albertlast]

Description

PackageManager::browse() cannot render on a forum that has never fetched the
version list from simplemachines.org:

in_array(): Argument #2 ($haystack) must be of type array, string given

The whole Package Manager → Browse Packages page is replaced by that error.

The emulation version list is fetched from simplemachines.org and cached in
admin_info_files. browse() reads it and passes it straight on:

$items = Utils::jsonDecode($data['data'], true);

Utils::$context['emulation_versions'] = preg_replace('~^SMF ~', '', $items);

Utils::jsonDecode() opens with if (empty($json)) { return null; }, so with
nothing cached $items is null, preg_replace() returns null rather than an
array, and in_array() on the next line rejects it.

Every fresh install is in that state until the first successful fetch, and any
forum without outbound access is in it permanently. On mine all four rows of
admin_info_files are zero-length:

filename              len
current-version.js    0
detailed-version.js   0
latest-news.js        0
latest-versions.txt   0

Falls back to an empty list. The three lines below already cope with that — they
add the running version to the list if it is not in it — so the dropdown offers
the current version rather than nothing.

$data also gets a ?? '', since fetch_assoc() returns false when there is no
row at all rather than a row with an empty column.

Testing

On a clean install of release-3.0 (a7ac468b1), MySQL, with no outbound access:

• Before: ?action=admin;area=packages;sa=browse shows the error above, and the
  page has only the "Package Manager" heading.
• After: the page renders in full — "Obtaining Packages" and "Emulation Support"
  sections, the advanced panel, and the emulation dropdown offering
  3.0 Alpha 4, selected.

composer lint is clean.

Found while testing the package browser for the #7933 split.

Issues References (Fixes|Related|Closes)

Related to #7933