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