Refactor: Use O(1) dereferencing for string emptiness checks instead of strlen()

[ismaileneskucuk]

Using strlen(str) > 0 to check for empty strings incurs an unnecessary O(N) cost, especially inside node rendering functions where it might be called repeatedly.

Checking the first character str[0] is the idiomatic C approach for an O(1) emptiness check.

This PR updates src/commonmark.c and src/main.c to use this safer and more performant approach, preventing potential performance bottlenecks on large documents with extensive attributes.

[jgm]

This makes sense to me, thanks. @nwellnhof any objections?

[nwellnhof]

Looks good to me. Unrelated, but you have to check errno after calling strtol to detect invalid and out-of-range numbers reliably.