Rename program to `test_write_null`

Rename program `test_write_eddy` to `test_write_null`. Following the
change of name of the file.

test_write_null.f90

-program test_write_eddy
+program test_write_null
 
   ! This program is a performance test for output of a shapefile
   ! collection. It writes null shapes.
@@ -100,4 +100,4 @@ program test_write_eddy
 
   CALL shpc_close(hshpc)
 
-end program test_write_eddy
+end program test_write_null