fix(cmake): Protect CMAKE_SYSTEM_VERSION to avoid empty values when cross-building

[uilianries]

Hello!

As explained on #1716, the missing quotes when comparing values and one is missing (CMAKE_SYSTEM_VERSION in this case), will fail CMake.

Also, quoting the book "Professional CMake - A Practical Guide":

Only use an unquoted argument if it is guaranteed that a variable by that name exists.

Professional CMake - Graig Scott, page 57 - The if() command

This PR guards CMAKE_SYSTEM_VERSION, so if it's empty, that if condition will not break.

fix #1716

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[supervacuus]

LGTM, Thanks! Please still add a change-log entry since this is certainly a publicly visible fix.

[supervacuus]

@jpnurmi, Warden fails because this is coming from a fork. No need to rerun. Warden failure is not a merge blocker.

[jpnurmi]

@uilianries Thanks for the fix! Could you add an entry to CHANGELOG.md?

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

**Fixes**:

- Protect CMAKE_SYSTEM_VERSION to avoid empty values when cross-building. ([#1720](https://github.com/getsentry/sentry-native/pull/1720))

[uilianries]

@jpnurmi Done, changelog updated. Nice to see you are using keepachangelog format! 😄