MIDEAST STOCKS-Saudi shares drop despite oil over $50 but Dubai lifted

* Saudi bourse falls as investors lack encouragement to build positions * But petrochemical stocks gain as oil breaks $50 * Dubai index gains for fourth session in a row * Dubai Parks surges over 6 pct after raising share capital * Egypt slips as local traders exit stock market By Celine Aswad DUBAI, May 26 (Reuters) - A break in Brent crude prices over $50 a barrel failed to support Saudi Arabia's index on Thursday, but helped carry Dubai's index higher for a fourth session. Other markets were