Improve zero check consistency in Short Weierstrass projective coordinates

[viktorking7]

Description

Replace direct field equality comparison with specialized is_zero() method in the Projective<P>::is_zero() implementation.

Before:

fn is_zero(&self) -> bool {
    self.z == P::BaseField::ZERO
}

After:

fn is_zero(&self) -> bool {
    self.z.is_zero()
}

Benefits

• Consistency: Aligns with standard field zero-checking patterns across the codebase
• Clarity: Makes the intent of zero comparison explicit
• Performance: Leverages optimized field-specific is_zero() implementations that may use constant-time operations