fix(block): don't draw the tag for empty constant attributes

A constant ATTDEF renders as block content, but with an empty default
value there is nothing to draw — the ATTDEF tessellator would fall back
to its tag-placeholder preview (meant for standalone definitions), so
blocks showed the literal tag text where AutoCAD shows nothing.

Co-Authored-By: Claude Fable 5 <[email protected]>

src/scene/cache/block_cache.rs

@@ -348,8 +348,13 @@ fn build_defn(
             // ATTRIB with the real value (tessellated separately). A CONSTANT
             // attribute has no ATTRIB; its value lives in the block itself, so
             // it must render as part of the block content (unless flagged
-            // invisible).
-            EntityType::AttributeDefinition(ad) if !ad.flags.constant || ad.flags.invisible => {
+            // invisible). With an empty value there is nothing to draw: the
+            // ATTDEF tessellator would fall back to its tag-placeholder
+            // preview, which belongs to a standalone definition, not to
+            // placed block content.
+            EntityType::AttributeDefinition(ad)
+                if !ad.flags.constant || ad.flags.invisible || ad.default_value.is_empty() =>
+            {
                 continue
             }
             EntityType::Insert(nested_ins) => {