Semantics: 'invocations' defaults to 1, not 0.
'invocations' was using 0 as the "shader never set" flag and as the
default. Formalize this and explicit set to 1 and link time.
diff --git a/Test/baseResults/410.geom.out b/Test/baseResults/410.geom.out
index b1f5a57..6f7252c 100644
--- a/Test/baseResults/410.geom.out
+++ b/Test/baseResults/410.geom.out
@@ -12,7 +12,7 @@
Shader version: 410
-invocations = 0
+invocations = -1
max_vertices = -1
input primitive = none
output primitive = none
@@ -65,7 +65,7 @@
ERROR: Linking geometry stage: At least one shader must specify a layout(max_vertices = value)
Shader version: 410
-invocations = 0
+invocations = 1
max_vertices = -1
input primitive = none
output primitive = none