GenericTable: prevent passing NULL to strtotime

Thumbnails are normally created on demand, e.g. when processing the format codes in a post's body text.
Normally, the temporary URL is only used once to generate thumbnails ad-hoc. However, when cache is
enabled, a reference to the asset may be used in a cached version of a formatted body text, skipping
the normal thumbnail generation routine.

When an asset is replaced, currently, all thumbnails are removed and references to them are removed
from the database. In case the asset is still referenced in a cached formatted body text, this could lead
to an error when requesting the thumbnail, as the thumbnail request is no longer present in the system.

As we do not know what posts use particular assets at this point in the code, it is best to work around this
issue by unsetting the thumbnail filenames rather than deleting the entries outright. This effectively
generates them again on the next request.

In the future, we should aim to keep track of what posts make use of assets, so cache may be invalidated
in a more targeted way.

models/Asset.php

@@ -606,14 +606,12 @@ class Asset
 			FROM assets_tags AS t
 			INNER JOIN assets AS a ON a.id_asset = t.id_asset
 			WHERE t.id_tag = {int:id_tag} AND
-				a.date_captured <= {datetime:date_captured} AND
-				a.id_asset != {int:id_asset}
-			ORDER BY a.date_captured DESC'
+				(a.date_captured, a.id_asset) < ({datetime:date_captured}, {int:id_asset})
+			ORDER BY a.date_captured DESC, a.id_asset DESC'
 			: '
 			FROM assets AS a
-			WHERE date_captured >= {datetime:date_captured} AND
-				a.id_asset != {int:id_asset}
-			ORDER BY date_captured ASC')
+			WHERE (a.date_captured, a.id_asset) > ({datetime:date_captured}, {int:id_asset})
+			ORDER BY date_captured ASC, a.id_asset ASC')
 			 . '
 			LIMIT 1',
 			[
@@ -639,14 +637,12 @@ class Asset
 			FROM assets_tags AS t
 			INNER JOIN assets AS a ON a.id_asset = t.id_asset
 			WHERE t.id_tag = {int:id_tag} AND
-				a.date_captured >= {datetime:date_captured} AND
-				a.id_asset != {int:id_asset}
-			ORDER BY a.date_captured ASC'
+				(a.date_captured, a.id_asset) > ({datetime:date_captured}, {int:id_asset})
+			ORDER BY a.date_captured ASC, a.id_asset ASC'
 			: '
 			FROM assets AS a
-			WHERE date_captured <= {datetime:date_captured} AND
-				a.id_asset != {int:id_asset}
-			ORDER BY date_captured DESC')
+			WHERE (a.date_captured, a.id_asset) < ({datetime:date_captured}, {int:id_asset})
+			ORDER BY date_captured DESC, a.id_asset DESC')
 			 . '
 			LIMIT 1',
 			[