fix: empty template on one line or no template tag causes error

Having an empty template on one line or no tempate tag at all would
incorrectly fallback to the deprecated template format where template
tags were ommited.

See: spacetelescope/jdaviz#2668
and widgetti/solara#625

Author: mariobuikhuizen

js/src/VueTemplateRenderer.js

@@ -102,7 +102,9 @@ function createComponentObject(model, parentView) {
             ...createFullVueComponents(fullVueComponents),
         },
         computed: { ...vuefile.SCRIPT && vuefile.SCRIPT.computed, ...aliasRefProps(model) },
-        template: vuefile.TEMPLATE || template,
+        template: vuefile.TEMPLATE === undefined && vuefile.SCRIPT === undefined && vuefile.STYLE === undefined
+            ? template
+            : vuefile.TEMPLATE,
         beforeMount() {
             callVueFn('beforeMount', this);
         },