Add test case for invalid src block.

org_mode_samples/lesser_block/src_with_no_data.org

@@ -0,0 +1,3 @@
+#+begin_src
+  echo "this is a source block."
+#+end_src

src/parser/lesser_block.rs

@@ -212,7 +212,7 @@ pub fn export_block<'r, 's>(
 #[tracing::instrument(ret, level = "debug")]
 pub fn src_block<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'s str, SrcBlock<'s>> {
     let (remaining, name) = lesser_block_begin("src")(context, input)?;
-    // TODO: DATA is mandatory and must follow the LANGUAGE SWITCHES ARGUMENTS pattern
+    // https://orgmode.org/worg/org-syntax.html#Blocks claims that data is mandatory and must follow the LANGUAGE SWITCHES ARGUMENTS pattern but testing has shown that no data and incorrect data here will still parse to a src block.
     let (remaining, parameters) = opt(tuple((space1, data)))(remaining)?;
     let (remaining, _nl) = line_ending(remaining)?;
     let lesser_block_end_specialized = lesser_block_end("src");