Preserve empty location for empty files

Without a region, error messages in exceptions are unable to print
a filename.

By retaining the location in a zero-length region and detecting this
when formatting the exception text it is possible to print the filename
and explicitly state that the file is completely empty.

Fixes #160

README.md

@@ -1943,6 +1943,8 @@ I appreciate the help of the contributors who introduced the great feature to th
   - Add missing include files
 - sneakypete81 (@sneakypete81)
   - Fix typo in error message
+- Oliver Kahrmann (@founderio)
+  - Fix missing filename in error message if parsed file is empty
 
 
 ## Licensing terms

toml/parser.hpp

@@ -1977,7 +1977,9 @@ result<Value, std::string> parse_toml_file(location& loc)
     const auto first = loc.iter();
     if(first == loc.end())
     {
-        return ok(value_type(table_type{} /*, empty file has no region ...*/));
+        // For empty files, return an empty table with an empty region (zero-length).
+        // Without the region, error messages would miss the filename.
+        return ok(value_type(table_type{}, region(loc, first, first), {}));
     }
 
     // put the first line as a region of a file

toml/value.hpp

@@ -66,9 +66,20 @@ throw_key_not_found_error(const Value& v, const key& ky)
     // ```
     // It actually points to the top-level table at the first character,
     // not `[table]`. But it is too confusing. To avoid the confusion, the error
-    // message should explicitly say "key not found in the top-level table".
+    // message should explicitly say "key not found in the top-level table",
+    // or "the parsed file is empty" if there is no content at all (0 bytes in file).
     const auto loc = v.location();
-    if(loc.line() == 1 && loc.region() == 1)
+    if(loc.line() == 1 && loc.region() == 0)
+    {
+        // First line with a zero-length region means "empty file".
+        // The region will be generated at `parse_toml_file` function
+        // if the file contains no bytes.
+        throw std::out_of_range(format_underline(concat_to_string(
+            "key \"", ky, "\" not found in the top-level table"), {
+                {loc, "the parsed file is empty"}
+            }));
+    }
+    else if(loc.line() == 1 && loc.region() == 1)
     {
         // Here it assumes that top-level table starts at the first character.
         // The region corresponds to the top-level table will be generated at