Well, I would say that it is important to have the conditional on it's own line. It makes is really quick and simple to see what the condition is.

eg: the condition is easier to read quickly in the second example.
<pre>
if (elementToCheck.hasSomeProperty("propName")) {
    elementToCheck.replacePropertyValue("propName", newVal)
    elementToCheck.doThings()
}
</pre>

<pre>
if (elementToCheck.hasSomeProperty("propName")) {

    elementToCheck.replacePropertyValue("propName", newVal)
    elementToCheck.doThings()
}
</pre>

And another reason is that the code in the conditional might *not* be directly related to the conditional. As an example in normal code I would have a blank line between logging and functional code.
<pre>
log.debug("Starting to process things")
                                       // I would have a blank line between logging and functional code
someObject.doThings()
</pre>

So it makes sense that you'd have blank line between a conditional and unrelated code.
<pre>
if (elementToCheck.hasSomeProperty()) {
                                           // And I would expect to have a blank line here as the condition is unrelated to the logging
    log.debug("Going down some code path")

    elementToCheck.doThings()
}
</pre>

And because it is a good idea to apply code-conventions consistently I would not suggest that you either always have a blank line after a conditional, or never. So even when it might not seem like there's value (as in this case) you will get value in other cases and at least the convention is applied consistently.