fix(b-table, b-table-lite): handle edge case where field slot returns no vNodes (fixes #3919)

[tmorehouse]

Describe the PR

Fixes an edge case where user conditionally renders content inside a scoped slot, which was causing the cell to be rendered with the raw value when the slot returned no vNodes. i.e. a v-if with no v-else:

  <template slot="cell[foo]" slot-scope="{ value }">
     <template v-if="value !== 0">
       {{ value }}
     </template>
  </template>

Fixes #3919

PR checklist

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Enhancement
• ARIA accessibility
• Documentation update
• Other (please describe)

Does this PR introduce a breaking change? (check one)

• No
• Yes (please describe)

The PR fulfills these requirements:

• It's submitted to the dev branch, not the master branch
• When resolving a specific issue, it's referenced in the PR's title (i.e. [...] (fixes #xxx[,#xxx]), where "xxx" is the issue number)
• It should address only one issue or feature. If adding multiple features or fixing a bug and adding a new feature, break them into separate PRs if at all possible.
• The title should follow the Conventional Commits naming convention (i.e. fix(alert): not alerting during SSR render, docs(badge): update pill examples, fix typos, chore: fix typo in README, etc). This is very important, as the CHANGELOG is generated from these messages.

If new features/enhancement/fixes are added or changed:

• Includes documentation updates (including updating the component's package.json for slot and event changes)
• Includes any needed TypeScript declaration file updates
• New/updated tests are included and passing (if required)
• Existing test suites are passing
• The changes have not impacted the functionality of other components or directives
• ARIA Accessibility has been taken into consideration (Does it affect screen reader users or keyboard only users? Clickable items should be in the tab index, etc.)

If adding a new feature, or changing the functionality of an existing feature, the PR's
description above includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

[codecov]

Codecov Report

Merging #3920 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #3920      +/-   ##
==========================================
+ Coverage   99.33%   99.33%   +<.01%     
==========================================
  Files         233      233              
  Lines        4493     4495       +2     
  Branches     1272     1271       -1     
==========================================
+ Hits         4463     4465       +2     
  Misses         24       24              
  Partials        6        6

Impacted Files	Coverage Δ
src/components/table/helpers/mixin-thead.js	100% <100%> (ø)	⬆️
src/components/table/helpers/mixin-tbody-row.js	100% <100%> (ø)	⬆️

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