fix: fix parsing of font sizes for pdfs with no headings (#155)

Author: ThomasDelsart

src/raglite/_markdown.py

@@ -70,7 +70,7 @@ def extract_font_size(span: dict[str, Any]) -> float:
                         span_font_size = extract_font_size(span)
                         if span_font_size < mode_font_size:
                             idx = 7
-                        elif span_font_size == mode_font_size:
+                        elif span_font_size == mode_font_size or len(heading_font_sizes) == 0:
                             idx = 6
                         else:
                             idx = np.argmin(np.abs(heading_font_sizes - span_font_size))  # type: ignore[assignment]