Fix SparseFileContent for delete-only patches

If an edit list is only deletion regions, the B side will wind up
with an empty range as there is no new content to include in the
file.  This is a corner case that means we have to always return
false from contains(int) when there is no content.

Change-Id: I745fa9ce2071acc2aa83bd84909477afeef0c3d4
Signed-off-by: Shawn O. Pearce <sop@google.com>

gerrit-prettify/src/main/java/com/google/gerrit/prettify/common/SparseFileContent.java

@@ -170,6 +170,10 @@ public class SparseFileContent {
 
     // Binary search for the range, since we know its a sorted list.
     //
+    if (ranges.isEmpty()) {
+      return null;
+    }
+
     int low = 0;
     do {
       final int mid = (low + high) / 2;