Support zero skipped text in OrgSource slicing.

src/parser/org_source.rs

@@ -145,6 +145,9 @@ where
         if new_end > self.end {
             panic!("Attempted to extend past the end of the WrappedInput.")
         }
+        if new_start == self.start && new_end == self.end {
+            return self.clone();
+        }
 
         let skipped_text = &self.full_source[self.start..new_start];
         let mut start_of_line = self.start_of_line;
@@ -183,7 +186,7 @@ where
             start: new_start,
             end: new_end,
             start_of_line,
-            preceding_character: skipped_text.chars().last(),
+            preceding_character: skipped_text.chars().last().or(self.preceding_character),
             bracket_depth,
             brace_depth,
             parenthesis_depth,

src/parser/util.rs

@@ -153,7 +153,7 @@ fn _preceded_by_whitespace<'s>(
         .unwrap_or(allow_start_of_file)
     {
         return Err(nom::Err::Error(CustomError::MyError(MyError(
-            "Must be preceded by a non-whitespace character.".into(),
+            "Must be preceded by a whitespace character.".into(),
         ))));
     }
     Ok((input, ()))