Fix exception wrapping blank line in index (#587)

For normal wrapping, the blank line is taken as a paragraph
so is not itself wrapped. However, because indexes have
been formatted into single whole lines, they are wrapped
one line at a time. This accidentally included wrapping the
blank lines. The wrapping routine hadn't been written to
expect wrapping an empty string.

1. In lower wrapping routine, deal with an empty paragraph
without raising an exception.
2. In upper index wrapping routine, only wrap lines that
have text

Finally, while testing, I found an independent bug (of mine)
which probably wouldn't occur in RL. I made a small index
by deleting most of the lines from the example supplied,
which left some page marks at the start of the closing
`I/` line. It then failed to recognize this as the end of the
index, and said "no match for opening `/I`". Fixed.

src/guiguts/maintext.py

@@ -2323,7 +2323,7 @@ def rewrap_section(
 
                     # Find matching close markup within section being wrapped
                     if close_index := self.search(
-                        rf"^{re.escape(block_type)}/\s*$",
+                        rf"^{PAGEMARK_PIN}*{re.escape(block_type)}/\s*$",
                         line_start,
                         stopindex=WRAP_END_MARK,
                         nocase=True,
@@ -2560,6 +2560,10 @@ def wrap_index_block(
         while self.compare(line_start, "<", INDEX_END_MARK):
             line_end = self.index(f"{line_start} lineend")
             line = self.get(line_start, line_end).rstrip()
+            # No need to wrap blank lines - just move on to the next l
+            if line == "":
+                line_start = self.index(f"{line_start}+1l")
+                continue
             # Don't include pagemark pins in calculations, since removed after wrapping
             line_no_pin = line.replace(PAGEMARK_PIN, "")
             match = re.match(r"( +)", line_no_pin)

src/guiguts/utilities.py

@@ -624,6 +624,10 @@ def process(line: str) -> None:
         # Executable section of enclosing function 'fill'.
         ####
 
+        # Empty paragraphs don't need wrapping
+        if paragraph == "":
+            return ""
+
         # Split the passed in paragraph into lines and add them
         # to the global list 'from_lines'.
         paragraph_to_lines(paragraph)