fix(loading): Prevent isLoading from being true with fallbackData

[Conqxeror]

This PR addresses an issue where isLoading was incorrectly set to true even when fallbackData was provided, particularly in SSR scenarios. This led to a "flash of skeleton" UI, which is undesirable when initial data is already available.

The change modifies the logic to ensure that isLoading remains false when fallbackData is present, while isValidating correctly reflects the background revalidation process. This allows the initial server-cached data to be displayed immediately, providing a smoother user experience.

Fixes #3046

[codesandbox-ci[bot]]

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[shuding]

Would love to have a test case for this, thanks!