hclparser: strip out blocks for json files

Body.JustAttributes cannot distinguish between blocks and attributes for
JSON files, so the variable block could be included in the list of
attributes returned.

This patch ensures that JSON and HCL files behave the same way by
removing all known block types first, from the provided config schema
and then from a generated definitions schema.

Fixes #1051

Signed-off-by: Justin Chadwell <me@jedevc.com>

bake/hcl_test.go

@@ -745,3 +745,13 @@ target "two" {
 	require.Equal(t, c.Targets[1].Name, "two")
 	require.Equal(t, map[string]string{"b": "pre-jkl"}, c.Targets[1].Args)
 }
+
+func TestEmptyVariableJSON(t *testing.T) {
+	dt := []byte(`{
+	  "variable": {
+	    "VAR": {}
+	  }
+	}`)
+	_, err := ParseFile(dt, "docker-bake.json")
+	require.NoError(t, err)
+}