STYLE: Remove redundant input check from ExtractImageFilter

[N-Dekker]

ExtractImageFilter::GenerateOutputInformation() has returned already when the input is null. So the "cannot cast input" exception at the end of the member function will never occur anymore.

• Follow-up to pull request COMP: Ubuntu2404 fixes issue 4607 #4608 commit 6650d2c
  "STYLE: Simplify code to remove unneeded dynamic_cast", Hans Johnson (@hjmjohnson), May 1, 2024

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🤓 When reviewing, it may be helpful to ignore whitespace changes: https://github.com/InsightSoftwareConsortium/ITK/pull/5319/files?w=1

In the original code, you can see that the member function already does return earlier, when the input is null:

ITK/Modules/Core/Common/include/itkExtractImageFilter.hxx

Lines 109 to 120 in 375ee8c

	const typename Superclass::InputImageConstPointer inputPtr = this->GetInput();
	if (!outputPtr || !inputPtr)
	{
	return;
	}
	// Set the output image size to the same value as the extraction region.
	outputPtr->SetLargestPossibleRegion(m_OutputImageRegion);
	// Set the output spacing and origin
	if (this->GetInput())