Revert "Fix heatmap non-color rendering to leave fully invalid cells blank"

This reverts commit 1406e06.

Author: kojix2

src/unicode_plot/graphics/heatmap_graphics.cr

@@ -129,12 +129,8 @@ module UnicodePlot
             io << ' '
           end
         else
-          # No color: keep missing cells blank just like the color path.
-          if fg != INVALID_COLOR || bg != INVALID_COLOR
-            io << HALF_BLOCK
-          else
-            io << ' '
-          end
+          # No color: just output the character (▄) so strip-ANSI comparison works
+          io << HALF_BLOCK
         end
       end
     end