Bug 66425: Avoid exceptions found via poi-fuzz

Prevent a NullPointerException

Fixes https://issues.oss-fuzz.com/issues/379574870

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1923059 13f79535-47bb-0310-9956-ffa450edef68

poi/src/main/java/org/apache/poi/sl/extractor/SlideShowExtractor.java

@@ -364,6 +364,10 @@ public class SlideShowExtractor<
         // PowerPoint seems to store files with \r as the line break
         // The messes things up on everything but a Mac, so translate them to \n
         String txt = tr.getRawText();
+        if (txt == null) {
+            return "";
+        }
+
         txt = txt.replace('\r', '\n');
         txt = txt.replace((char) 0x0B, sep);