Handling of reads at EOF, consistent with stdlib.

[ioquatix]

Fixed EOF behavior in read to match Ruby's standard IO semantics. Previously, both read() and read(size) would return nil at EOF. Now read() without arguments correctly returns an empty string "" at EOF, while read(size) with a specific size returns nil at EOF, exactly matching Ruby's IO#read behavior.

Types of Changes

• Bug fix.

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.