chore: improve cards compatibility with steps

[imfing]

for use cases such as:

{{% steps %}}

### Step 1

This is the second step.

{{< cards >}}
  {{< card link="../callout" title="Callout" icon="warning" >}}
  {{< card link="/" title="No Icon" >}}
{{< /cards >}}


### Step 2

This is the second step.

### Step 3

This is the third step.

{{% /steps %}}

since empty lines will be treated as Markdown thus generating unwanted <p> tag that messes up the HTML

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