Align <hx-partial> and hx-swap-oob behavior on empty main swap

[scriptogre]

Why

Main target:

<div id="main-target">Original</div>

hx-swap-oob response:

<div id="oob-target" hx-swap-oob="true">Updated</div>

Result:

<div id="main-target"></div>

<hx-partial> response:

<hx-partial hx-target="#partial-target">Updated</hx-partial>

Result:

<div id="main-target">Original</div>

swapEmpty currently has three states:

• true clears the main target.
• false keeps the main target.
• undefined clears it for hx-swap-oob, but keeps it for <hx-partial>.

The special undefined state is confusing.

Change

Default defaultSwapEmpty to true, so both responses clear the main target (like hx-swap-oob does now), so they at least work consistently.

[1cg]

Per discussion in chat, going to keep the asymmetry just needs docs