Fix psalm issues in theming app

After this change, we are down to only one psalm warning for this app
and related to the Application.php. This also make composer
psam:update-baseline not silently ignore new errors.

Signed-off-by: Carl Schwan <carl@carlschwan.eu>

1 files changed, 1 insertions, 0 deletions

diff --git a/apps/theming/lib/Command/UpdateConfig.php b/apps/theming/lib/Command/UpdateConfig.php
index 1ff75b5ba70..bb226668943 100644
--- a/apps/theming/lib/Command/UpdateConfig.php
+++ b/apps/theming/lib/Command/UpdateConfig.php
@@ -79,6 +79,7 @@ class UpdateConfig extends Command {
 	protected function execute(InputInterface $input, OutputInterface $output): int {
 		$key = $input->getArgument('key');
 		$value = $input->getArgument('value');
+		assert(is_string($value) || $value === null, 'At most one value should be provided.');
 
 		if ($key === null) {
 			$output->writeln('Current theming config:');