fix: insert context startpage for proper update

[enjeck]

There's a bug with updating context. To reproduce:

• Create a context without any tables/views
• Save context
• Edit context and add a table or view
• Try to save. There's an error.

Error Trace

                "OCA\\Tables\\Controller\\ContextController",
                "update",
                [
                   "OC\\AppFramework\\DependencyInjection\\DIContainer"
                ],
                [
                   "1",
                   "ocs.tables.context.update"
                ]
             ]
          },
          {
             "file":"/var/www/html/ocs/v1.php",
             "line":43,
             "function":"match",
             "class":"OC\\Route\\Router",
             "type":"->",
             "args":[
                "/ocsapp/apps/tables/api/2/contexts/1"
             ]
          },
          {
             "file":"/var/www/html/ocs/v2.php",
             "line":7,
             "args":[
                "/var/www/html/ocs/v1.php"
             ],
             "function":"require_once"
          }
       ],
       "File":"/var/www/html/apps/tables/lib/Db/PageContentMapper.php",
       "Line":37
    },
    "message":"OCA\\Tables\\Db\\PageContentMapper::findByPageAndNodeRelation(): Argument #1 ($pageId) must be of type int, null given, called in /var/www/html/apps/tables/lib/Service/ContextService.php on line 365 in file '/var/www/html/apps/tables/lib/Db/PageContentMapper.php' line 37",
    "exception":{
       
    },
    "CustomMessage":"OCA\\Tables\\Db\\PageContentMapper::findByPageAndNodeRelation(): Argument #1 ($pageId) must be of type int, null given, called in /var/www/html/apps/tables/lib/Service/ContextService.php on line 365 in file '/var/www/html/apps/tables/lib/Db/PageContentMapper.php' line 37"
 }
}

The problem occured because there's no startpage inserted when an "empty" context without resources is created.

[enjeck]

An error comes up when a new context without any resources is created

foreach() argument must be of type array|object, null given at /var/www/html/apps/tables/lib/Service/ContextService.php#433"