Blank lines with spaces can render unexpected setext headers

[jldec]

Marked version:
4.2.3 and later

Describe the bug
According to commonmark setext headers should be generated when text is not interrupted by a blank line.

Since marked v4.2.3, lines containing one or more spaces are not being treated as blank lines in setext headers. This causes surprising H1 anc H2 headings where there should be paragraphs.

To Reproduce
v4.2.2 works
v4.2.3 and later - surprise 😮
commonmark confirmed

Expected behavior
Match behavior of commonmark setext headers.