fix(b-table, b-table-lite): handle edge case where field slot returns…

… no vNodes (fixes #3919) (#3920)

src/components/table/helpers/mixin-tbody-row.js

@@ -204,7 +204,9 @@ export default {
       // a square bracket and if using in-document HTML templates, the
       // v-slot attributes are lower-cased by the browser.
       const slotNames = [`cell[${key}]`, `cell[${key.toLowerCase()}]`, 'cell[]']
-      let $childNodes = this.normalizeSlot(slotNames, slotScope) || toString(formatted)
+      let $childNodes = this.hasNormalizedSlot(slotNames)
+        ? this.normalizeSlot(slotNames, slotScope)
+        : toString(formatted)
       if (this.isStacked) {
         // We wrap in a DIV to ensure rendered as a single cell when visually stacked!
         $childNodes = [h('div', {}, [$childNodes])]

src/components/table/helpers/mixin-thead.js

@@ -116,20 +116,22 @@ export default {
             ...slotNames
           ]
         }
-        const slot = this.normalizeSlot(slotNames, {
-          label: field.label,
-          column: field.key,
-          field,
-          isFoot,
-          // Add in row select methods
-          selectAllRows,
-          clearSelected
-        })
-        if (!slot) {
-          // need to check if this will work
+        const hasSlot = this.hasNormalizedSlot(slotNames)
+        let slot = field.label
+        if (hasSlot) {
+          slot = this.normalizeSlot(slotNames, {
+            label: field.label,
+            column: field.key,
+            field,
+            isFoot,
+            // Add in row select methods
+            selectAllRows,
+            clearSelected
+          })
+        } else {
           data.domProps = htmlOrText(field.labelHtml)
         }
-        return h(BTh, data, slot || field.label)
+        return h(BTh, data, slot)
       }
 
       // Generate the array of <th> cells