thema: Disable schema emptiness invariants

Attempting to migrate grafana/grafana to the new flattened lineage
format has revealed that there were a number of existing lineages that
contained no actual fields - only definitions. This violates the schema
non-emptiness invariant introduced during lineage flattening.

These lineages will be refactored, but to ease the transition, this
commit temporarily disables the schema non-emptiness invariant in thema
so that it can be handled gradually, rather than right away and up
front. This does not meaningfully degrade thema's guarantees, as we're
just returning to where it was pre-flattening.

lineage.cue

@@ -27,7 +27,8 @@ import (
 	//
 	// A lineage's joinSchema must never change as the lineage evolves.
 
-	joinSchema?: struct.MinFields(1)
+	//	joinSchema?: struct.MinFields(1)
+	joinSchema: _
 
 	// schemas is the ordered list of all schemas in the lineage.
 	//
@@ -256,7 +257,7 @@ import (
 
 	_#schema: _join & schema
 
-	_schemaIsNonEmpty: struct.MinFields(1) & _#schema
+	//	_schemaIsNonEmpty: struct.MinFields(1) & _#schema
 
 	// examples is an optional set of named examples of the schema, intended
 	// for use in documentation or other non-functional contexts.

lineage_test.go

@@ -57,6 +57,7 @@ func TestInvalidLineages(t *testing.T) {
 		ToDo: map[string]string{
 			"invalidlineage/defaultchange": "Thema compat analyzer fails to classify changes to default values as breaking",
 			"invalidlineage/joindef":       "no invariant checker written to disallow definitions from joinSchema",
+			"invalidlineage/onlydef":       "Lineage schema non-emptiness constraints are temporarily suspended while migrating grafana to flattened lineage structure",
 		},
 	}
 

testdata/invalidlineage/empty-schemas.txtar

@@ -6,8 +6,8 @@ thema.#Lineage
 name: "empty-schemas"
 schemas: []
 -- out/bindfail --
-schemas: incompatible list lengths (0 and 1)
+incompatible list lengths (0 and 1)
 _sortedSchemas: invalid value [] (does not satisfy list.MinItems(1)): len(list) < MinItems(1) (0 < 1):
-    ../../lineage.cue:97:7
-    ../../lineage.cue:93:21
-    ../../lineage.cue:97:21
+    ../../lineage.cue:98:7
+    ../../lineage.cue:94:21
+    ../../lineage.cue:98:21

testdata/invalidlineage/second-schema-versionless.txtar

@@ -32,8 +32,8 @@ lenses: [
 ]
 -- out/bindfail --
 _schemasAreOrdered.1: conflicting values true and false:
-    ../../lineage.cue:124:3
-    ../../lineage.cue:125:4
-    ../../lineage.cue:130:5
-    ../../lineage.cue:131:11
+    ../../lineage.cue:125:3
+    ../../lineage.cue:126:4
+    ../../lineage.cue:131:5
+    ../../lineage.cue:132:11
     ./in.cue:4:1