fix: skip output when stdin is empty

[LeonidasZhak]

Summary

When delta receives empty stdin (zero bytes), it now produces no output instead of emitting an ANSI reset sequence (\x1b[00;39m).

Problem

printf '' | delta emits a phantom blank line because the painter's output buffer contains an ANSI reset sequence that gets written even when no input is processed.

Solution

Added a has_input flag in StateMachine::consume() that tracks whether any lines were read. If no input was received, return early before the painter emits its output buffer.

Testing

# Before: emits ANSI reset
printf '' | delta | cat -v  # ^[[00;39m

# After: no output
printf '' | delta | cat -v  # (empty)

Fixes #2151