Fix missing dependency for approval tests

Author: aaltat

requirements-dev.txt

@@ -4,6 +4,7 @@ invoke >= 1.4.1
 mockito >= 1.2.2
 robotstatuschecker >= 2.0.3
 approvaltests == 3.1.0
+empty_files >= 0.0.3
 pytest >= 6.0.2
 pytest-mockito >= 0.0.4
 pytest-approvaltests >= 0.1.0