HB: Detail page for some schools empty

[cyroxx]

Some schools that are listed in the overview page of the "Schulwegweiser" have an empty detail page.

Examples:

• Neue Oberschule Lehe (554) https://www.bildung.bremen.de/detail.php?&gsid=bremen117.c.8954.de&Sid=554
• Wirtschaft-und Sozialakademie der Arbeitnehmerkammer Bremen Geschäftsbereich Bremerhaven (881) https://www.bildung.bremen.de/detail.php?&gsid=bremen117.c.8954.de&Sid=881
• Grundschule Stella Maris (855) https://www.bildung.bremen.de/detail.php?&gsid=bremen117.c.8954.de&Sid=855

This leads to an exception in the spiders' normalize method.

[k-nut]

We agreed that we want to get in touch with the website providers to find out why these pages are empty. For now we are going to skip the empty pages entirely as per PR #73.