fix: Fix redundant empty line in markdown output when no project counts are shown

[vdmgolub]

Caught this in failing GH actions tests: https://github.com/infracost/actions/actions/runs/4359407857/jobs/7621166205

Before:

💰 Infracost estimate: **monthly cost will increase by $586 (+73%) 📈**
<table>
  <thead>
    <td>Project</td>
    <td>Previous</td>
    <td>New</td>
    <td>Diff</td>
  </thead>
  <tbody>
    <tr>
      <td>infracost/actions/examples/multi-project-config-file/code/dev</td>
      <td align="right">$51.97</td>
      <td align="right">$77.37</td>
      <td>+$25.40 (+49%)</td>
    </tr>
    <tr>
      <td>infracost/actions/examples/multi-project-config-file/code/prod</td>
      <td align="right">$748</td>
      <td align="right">$1,308</td>
      <td>+$561 (+75%)</td>
    </tr>
    <tr>
      <td>All projects</td>
      <td align="right">$800</td>
      <td align="right">$1,386</td>
      <td>+$586 (+73%)</td>
    </tr>
  </tbody>
</table>


<details>
<summary><strong>Infracost output</strong></summary>

With the fix:

💰 Infracost estimate: **monthly cost will increase by $586 (+73%) 📈**
<table>
  <thead>
    <td>Project</td>
    <td>Previous</td>
    <td>New</td>
    <td>Diff</td>
  </thead>
  <tbody>
    <tr>
      <td>infracost/actions/examples/multi-project-config-file/code/dev</td>
      <td align="right">$51.97</td>
      <td align="right">$77.37</td>
      <td>+$25.40 (+49%)</td>
    </tr>
    <tr>
      <td>infracost/actions/examples/multi-project-config-file/code/prod</td>
      <td align="right">$748</td>
      <td align="right">$1,308</td>
      <td>+$561 (+75%)</td>
    </tr>
    <tr>
      <td>All projects</td>
      <td align="right">$800</td>
      <td align="right">$1,386</td>
      <td>+$586 (+73%)</td>
    </tr>
  </tbody>
</table>

<details>
<summary><strong>Infracost output</strong></summary>

With project counts:

💰 Infracost estimate: **monthly cost will increase by $561 (+70%) 📈**
<table>
  <thead>
    <td>Project</td>
    <td>Previous</td>
    <td>New</td>
    <td>Diff</td>
  </thead>
  <tbody>
    <tr>
      <td>infracost/actions/examples/multi-project-config-file/code/prod</td>
      <td align="right">$748</td>
      <td align="right">$1,308</td>
      <td>+$561 (+75%)</td>
    </tr>
    <tr>
      <td>All projects</td>
      <td align="right">$800</td>
      <td align="right">$1,360</td>
      <td>+$561 (+70%)</td>
    </tr>
  </tbody>
</table>

1 project has no cost estimate changes.

<details>
<summary><strong>Infracost output</strong></summary>