Spec.format: print false attributes if requested (#37932)

lib/spack/spack/spec.py

@@ -4333,7 +4333,7 @@ def write_attribute(spec, attribute, color):
 
                     if callable(current):
                         raise SpecFormatStringError("Attempted to format callable object")
-                    if not current:
+                    if current is None:
                         # We're not printing anything
                         return
 

lib/spack/spack/test/spec_semantics.py

@@ -660,6 +660,7 @@ def test_spec_formatting(self, default_mock_concretization):
             ("{architecture.os}", "", "os", lambda spec: spec.architecture),
             ("{architecture.target}", "", "target", lambda spec: spec.architecture),
             ("{prefix}", "", "prefix", lambda spec: spec),
+            ("{external}", "", "external", lambda spec: spec),  # test we print "False"
         ]
 
         hash_segments = [