Fix prefix path when loading Hydrogen's theme-element.json

This is needed when using a prefix path that doesn't end with a slash.

Signed-off-by: Sergio Durigan Junior <sergiodj@sergiodj.net>

roles/custom/matrix-client-hydrogen/templates/config.json.j2

@@ -3,7 +3,7 @@
 	"defaultHomeServer": {{ matrix_client_hydrogen_default_hs_url | string | to_json }},
 	"bugReportEndpointUrl": {{ matrix_client_hydrogen_bugReportEndpointUrl | to_json }},
 	"themeManifests": [
-		"{{ matrix_client_hydrogen_path_prefix }}assets/theme-element.json"
+		"{{ '' if matrix_client_hydrogen_path_prefix == '/' else matrix_client_hydrogen_path_prefix }}/assets/theme-element.json"
 	],
 	"defaultTheme": {
 		"light": "element-light",