Corrected the check_density.py script so that it works in the
absence of FIXED_BBOX.  It was attempting to "catch" the result
of "property FIXED_BBOX", but the command always returns a valid
result, which will be an empty string if the property is undefined.

sky130/custom/scripts/check_density.py

diff --git a/sky130/custom/scripts/check_density.py b/sky130/custom/scripts/check_density.py
index 72bf2f9..4358ed3 100755
--- a/sky130/custom/scripts/check_density.py
+++ b/sky130/custom/scripts/check_density.py
@@ -191,8 +191,11 @@
 
         print('select top cell', file=ofile)
         print('expand', file=ofile)
-        print('set fullbox [box values]', file=ofile)
         # Override with FIXED_BBOX, if it is defined
+        print('set fullbox [property FIXED_BBOX]', file=ofile)
+        print('if {$fullbox == ""} {', file=ofile)
+        print('    set fullbox [box values]', file=ofile)
+        print('}', file=ofile)
         print('catch {set fullbox [property FIXED_BBOX]}', file=ofile)
         print('set xmax [lindex $fullbox 2]', file=ofile)
         print('set xmin [lindex $fullbox 0]', file=ofile)