MDL-29835: filemanager - zero means zero, not unlimited

The form_filemanager constructor was using empty() rather than !isset(),
so overwrote maxfiles=0 with the default (-1).  In addition, the JS UI
treated 0 as unlimited - but the non-JS fallback UI treated it as zero.
This would result in a file manager which allowed files to be
chosen/uploaded, which would then vanish into the void when the backend
saw that the maximum number of files was 0.

lib/form/dndupload.js

@@ -537,7 +537,8 @@ M.form_dndupload.init = function(Y, options) {
             if (!overwrite) {
                 this.currentfilecount++;
             }
-            if (this.options.maxfiles > 0 && this.currentfilecount > this.options.maxfiles) {
+            // The value for "unlimited files" is -1, so 0 should mean 0.
+            if (this.options.maxfiles >= 0 && this.currentfilecount > this.options.maxfiles) {
                 // Too many files - abort entire upload.
                 this.uploadqueue = [];
                 this.renamequeue = [];

lib/form/filemanager.php

@@ -332,7 +332,8 @@ public function __construct(stdClass $options) {
             $defaults['defaultlicense'] = $CFG->sitedefaultlicense;
         }
         foreach ($defaults as $key=>$value) {
-            if (empty($options->$key)) {
+            // Using !isset() prevents us from overwriting falsey values with defaults (as empty() did).
+            if (!isset($options->$key)) {
                 $options->$key = $value;
             }
         }