Fix for HER-1962 NPE from missing sheet

* SheetOverlaysManager.java
    getOverlayMap(String) - return null if sheet missing instead of triggering npe
* KeyedProperties.java
    get(String) - check for null return value from getOverlayMap() and log warning

engine/src/main/java/org/archive/crawler/spring/SheetOverlaysManager.java

@@ -169,7 +169,12 @@ BeanFactoryAware, OverlayMapsSource, ApplicationListener {
      * @see org.archive.spring.OverlayMapsSource#getOverlayMap(java.lang.String)
      */
     public Map<String, Object> getOverlayMap(String name) {
-        return sheetsByName.get(name).getMap();
+        Sheet sheet = sheetsByName.get(name);
+        if (sheet != null) {
+            return sheet.getMap();
+        } else {
+            return null;
+        }
     }
 
     /**