Handle empty array case via ConstantArrayType::spliceArray()

[herndlm]

This basically acts as recursion guard because a bit later the following piece of code

		if ($offset === null || $length === null) {
			return $this->degradeToGeneralArray()
				->spliceArray($offsetType, $lengthType, $replacementType);
		}

would degrade the empty constant array to the same empty constant array, calling the same method again, which then leads to infinite recursion.
We have that exact same code for sliceArray() as well btw.

Closes phpstan/phpstan#13279

Was introduced in #3952

[ondrejmirtes]

Perfect, thank you!