[FIX] component: make sure default slot is applied

Before this commit, owl was erroneously defining default slots in most
cases, even though they are empty. The problem occurs when the content
of a component slots is a t-set-slot, and we remove that, then use the
rest as default slots, even though it is only reduced to a set of text
nodes.

With this commit, we only consider the content as default content if it
is not only a sequence of text nodes containing only spaces.

closes #882

src/component/directive.ts

@@ -452,12 +452,17 @@ QWeb.addDirective({
         }
       }
       if (clone.childNodes.length) {
+        let hasContent = false;
         const t = clone.ownerDocument!.createElement("t");
         for (let child of Object.values(clone.childNodes)) {
+          hasContent =
+            hasContent || (child instanceof Text ? Boolean(child.textContent.trim().length) : true);
           t.appendChild(child);
         }
-        const slotFn = qweb._compile(`slot_default_template`, { elem: t, hasParent: true });
-        QWeb.slots[`${slotId}_default`] = slotFn;
+        if (hasContent) {
+          const slotFn = qweb._compile(`slot_default_template`, { elem: t, hasParent: true });
+          QWeb.slots[`${slotId}_default`] = slotFn;
+        }
       }
     }