MDL-49329 admin: Fix report on required core version in missing plugins

If the plugin was missing from disk, the required core version was
displayed as "Moodle {$a}" because the core dependency resolver always
returns an object.

lib/classes/plugin_manager.php

@@ -794,6 +794,11 @@ class core_plugin_manager {
     public function resolve_requirements(\core\plugininfo\base $plugin, $moodleversion=null, $moodlebranch=null) {
         global $CFG;
 
+        if ($plugin->versiondisk === null) {
+            // Missing from disk, we have no version.php to read from.
+            return array();
+        }
+
         if ($moodleversion === null) {
             $moodleversion = $CFG->version;
         }

lib/tests/plugin_manager_test.php

@@ -350,6 +350,7 @@ class core_plugin_manager_testcase extends advanced_testcase {
         // Prepare a fake pluginfo instance.
         $pluginfo = testable_plugininfo_base::fake_plugin_instance('fake', '/dev/null', 'one', '/dev/null/fake',
             'testable_plugininfo_base', $pluginman);
+        $pluginfo->versiondisk = 2015060600;
 
         // Test no $plugin->requires is specified in version.php.
         $pluginfo->versionrequires = null;
@@ -409,6 +410,12 @@ class core_plugin_manager_testcase extends advanced_testcase {
         $pluginfo->dependencies = array('foo_bar' => 2025010100);
         $reqs = $pluginman->resolve_requirements($pluginfo, 2015110900, 30);
         $this->assertEquals($pluginman::REQUIREMENT_UNAVAILABLE, $reqs['foo_bar']->availability);
+
+        // Plugin missing from disk - no version.php available.
+        $pluginfo = testable_plugininfo_base::fake_plugin_instance('fake', '/dev/null', 'missing', '/dev/null/fake',
+            'testable_plugininfo_base', $pluginman);
+        $pluginfo->versiondisk = null;
+        $this->assertEmpty($pluginman->resolve_requirements($pluginfo, 2015110900, 30));
     }
 
     public function test_missing_dependencies() {
@@ -416,8 +423,11 @@ class core_plugin_manager_testcase extends advanced_testcase {
 
         $one = testable_plugininfo_base::fake_plugin_instance('fake', '/dev/null', 'one', '/dev/null/fake',
             'testable_plugininfo_base', $pluginman);
+        $one->versiondisk = 2015070800;
+
         $two = testable_plugininfo_base::fake_plugin_instance('fake', '/dev/null', 'two', '/dev/null/fake',
             'testable_plugininfo_base', $pluginman);
+        $two->versiondisk = 2015070900;
 
         $pluginman->inject_testable_plugininfo('fake', 'one', $one);
         $pluginman->inject_testable_plugininfo('fake', 'two', $two);