fix: early return from slideDown if duration is zero

[Kneemund]

This PR adds an early return to the slideDown function that skips the animation if the duration is set to zero.

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Currently, when the duration is zero, the second timeout runs BEFORE the first one (at least on Firefox), and doesn't properly clear the style attributes:

AdminLTE/src/ts/util/index.ts

Lines 96 to 112 in 5196a37

	globalThis.setTimeout(() => {
	target.style.boxSizing = 'border-box'
	target.style.transitionProperty = 'height, margin, padding'
	target.style.transitionDuration = `${duration}ms`
	target.style.height = `${height}px`
	target.style.removeProperty('padding-top')
	target.style.removeProperty('padding-bottom')
	target.style.removeProperty('margin-top')
	target.style.removeProperty('margin-bottom')
	}, 1)
	globalThis.setTimeout(() => {
	target.style.removeProperty('height')
	target.style.removeProperty('overflow')
	target.style.removeProperty('transition-duration')
	target.style.removeProperty('transition-property')
	}, duration)

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This fixes a bug in the sidebar treeview, where this function is used with a duration of zero to instantly open entries when the page loads:

AdminLTE/src/ts/treeview.ts

Line 120 in 5196a37

slideDown(childElement, 0)

As an alternative fix, the duration parameter could be clamped to be greater than 1.

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