Adapted test SCH references

- Now we keep title block entries, even if they are " "
  This is how KiCad behaves

tests/reference/6_0_4/deeper.kicad_sch

@@ -4,6 +4,10 @@
 
   (paper "A4" portrait)
 
+  (title_block
+    (date " ")
+  )
+
   (lib_symbols
     (symbol "Device:R" (pin_numbers hide) (pin_names (offset 0)) (in_bom yes) (on_board yes)
       (property "Reference" "R" (id 0) (at 2.032 0 90)

tests/reference/6_0_4/sub-sheet.kicad_sch

@@ -4,6 +4,10 @@
 
   (paper "User" 297.002 210.007)
 
+  (title_block
+    (date " ")
+  )
+
   (lib_symbols
     (symbol "74xx:74LS04" (in_bom yes) (on_board yes)
       (property "Reference" "U" (id 0) (at 0 1.27 0)