consider empty files (no ruby tokens) to have 0 lines

Author: alexrford

ql/src/codeql/files/FileSystem.qll

@@ -183,7 +183,11 @@ class File extends Container, @file {
   }
 
   /** Gets the number of lines in this file. */
-  int getNumberOfLines() { result = max(this.getAToken().getLocation().getEndLine()) }
+  int getNumberOfLines() {
+    if exists(this.getAToken())
+    then result = max(this.getAToken().getLocation().getEndLine())
+    else result = 0
+  }
 
   /** Gets the number of lines of code in this file. */
   int getNumberOfLinesOfCode() { result = count(int line | this.line(line, false)) }