fix(assert): prevent panic on nil condition in Eventually and Never

[AdamMagued]

Validate that condition != nil and tick > 0 at the start of Eventually, EventuallyWithT, and Never before creating timers or spawning worker goroutines.

Previously, passing a nil condition to Eventually, EventuallyWithT, or Never caused a panic inside a newly spawned goroutine when executing condition(). Because the panic occurred on a separate goroutine, it could not be recovered by the calling test function and caused the entire test binary to abort without reporting an assertion failure to TestingT. Similarly, passing a non-positive tick caused time.NewTicker to panic.

With this change:

• If condition is nil, a clean assertion failure is reported to TestingT with "Condition must not be nil" and the function returns false.
• If tick <= 0, an assertion failure is reported with "tick must be greater than 0" and the function returns false.
• Added regression tests in assert/assertions_test.go covering nil conditions and non-positive tick intervals for Eventually, EventuallyWithT, and Never.

Fixes #1970

[AdamMagued]

Closing in favor of issue author's PR #1971 to keep the review queue clean.