YANG parser BUGFIX parsing when statement with empty ("") argument

Adds test for when-stmt YANG  parser.

Applies also to some other statements, so not only when-stmt is fixed.

Fixes #805
diff --git a/src/tree_schema_free.c b/src/tree_schema_free.c
index 546b4d2..b7fe1a8 100644
--- a/src/tree_schema_free.c
+++ b/src/tree_schema_free.c
@@ -234,7 +234,7 @@
     FREE_ARRAY(ctx, grp->exts, lysp_ext_instance_free);
 }
 
-static void
+void
 lysp_when_free(struct ly_ctx *ctx, struct lysp_when *when)
 {
     FREE_STRING(ctx, when->cond);