Fixed KiCad 6 problems with empty title block items

- The saved SCH wasn't usable if company, rev, title, etc.
  were absent in the original file.

kibot/kicad/v6_sch.py

@@ -1576,12 +1576,17 @@ class SchematicV6(Schematic):
 
     def write_title_block(self):
         data = [Sep()]
-        data += [_symbol('title', [self.title_ori]), Sep()]
-        data += [_symbol('date', [self.date_ori]), Sep()]
-        data += [_symbol('rev', [self.revision_ori]), Sep()]
-        data += [_symbol('company', [self.company_ori]), Sep()]
+        if self.title_ori:
+            data += [_symbol('title', [self.title_ori]), Sep()]
+        if self.date_ori:
+            data += [_symbol('date', [self.date_ori]), Sep()]
+        if self.revision_ori:
+            data += [_symbol('rev', [self.revision_ori]), Sep()]
+        if self.company_ori:
+            data += [_symbol('company', [self.company_ori]), Sep()]
         for num, val in enumerate(self.comment_ori):
-            data += [_symbol('comment', [num+1, val]), Sep()]
+            if val:
+                data += [_symbol('comment', [num+1, val]), Sep()]
         return [Sep(), Sep(), _symbol('title_block', data)]
 
     def write_lib_symbols(self, cross=False):

tests/reference/6_0_2/deeper.kicad_sch

@@ -5,19 +5,6 @@
   (paper "A4")
 
   (title_block
-    (title "")
-    (date "")
-    (rev "")
-    (company "")
-    (comment 1 "")
-    (comment 2 "")
-    (comment 3 "")
-    (comment 4 "")
-    (comment 5 "")
-    (comment 6 "")
-    (comment 7 "")
-    (comment 8 "")
-    (comment 9 "")
   )
 
   (lib_symbols

tests/reference/6_0_2/sub-sheet.kicad_sch

@@ -5,19 +5,6 @@
   (paper "A4")
 
   (title_block
-    (title "")
-    (date "")
-    (rev "")
-    (company "")
-    (comment 1 "")
-    (comment 2 "")
-    (comment 3 "")
-    (comment 4 "")
-    (comment 5 "")
-    (comment 6 "")
-    (comment 7 "")
-    (comment 8 "")
-    (comment 9 "")
   )
 
   (lib_symbols

tests/reference/6_0_2/test_v5.kicad_sch

@@ -13,11 +13,6 @@
     (comment 2 "Comment 2")
     (comment 3 "Comment 3")
     (comment 4 "@Comment4@")
-    (comment 5 "")
-    (comment 6 "")
-    (comment 7 "")
-    (comment 8 "")
-    (comment 9 "")
   )
 
   (lib_symbols