Don't freak out if a plugin is missing its schema version

CHANGELOG-v3.md

@@ -13,6 +13,7 @@
 - The installer now checks `project.yaml` when determining the default site name, handle, base URL, and language values. ([#3661](https://github.com/craftcms/cms/issues/3661))
 - The Base URL field in the web-based installer now autouggests environment variable names and aliases.
 - Craft now creates a `.gitignore` file in the `storage/config-backups/` folder, preventing any other files within it from getting tracked by Git.
+- Craft no longer prevents changes in `project.yaml` from being synced if a plugins’ schema version in `project.yaml` doesn’t match up with its installed schema version, if one of them is blank.
 
 ### Fixed
 - Fixed a PHP notice that occurred when updating to Craft 3.1 if there were any plugins installed without settings.

src/migrations/Install.php

@@ -1090,7 +1090,7 @@ private function _installPlugins()
             $expectedSchemaVersion = $projectConfig->get(Plugins::CONFIG_PLUGINS_KEY . '.' . $handle . '.schemaVersion', true);
 
             /** @var Plugin|null $plugin */
-            if ($plugin->schemaVersion != $expectedSchemaVersion) {
+            if ($plugin->schemaVersion && $expectedSchemaVersion && $plugin->schemaVersion != $expectedSchemaVersion) {
                 throw new InvalidPluginException($handle, "{$handle} is installed at the wrong schema version ({$plugin->schemaVersion}, but project.yaml lists {$expectedSchemaVersion}).");
             }
         }