Skip cells missing an "r" cell reference

[bf4]

Creek skips cells that don't have r (cell reference) attributes like r="A7".

<!-- Expected format -->
<row r="7"><c r="A7" t="str"><v>12/12/2025</v></c>...</row>

<!-- Malformed format (missing r="A7") -->
<row r="7"><c t="str"><v>12/12/2025</v></c>...</row>

This causes data rows to appear empty while header rows (which typically have r attributes) parse correctly.