Preserve leading newlines when CMARK_OPT_PRESERVE_WHITESPACE is set

rdar://79148254

Author: jmschonfeld

api_test/main.c

@@ -1201,6 +1201,11 @@ static void preserve_whitespace_opt(test_batch_runner *runner) {
   check_markdown_plaintext(runner, "hel\nworld\nlo");
   check_markdown_plaintext(runner, " hel \n world \n lo ");
   check_markdown_plaintext(runner, "  hello \n  \n world  ");
+  check_markdown_plaintext(runner, "\n");
+  check_markdown_plaintext(runner, "\n\n\n");
+  check_markdown_plaintext(runner, "\nHello");
+  check_markdown_plaintext(runner, "Hello\n");
+  check_markdown_plaintext(runner, "\nHello\n");
 }
 
 static void check_markdown_attributes_node(test_batch_runner *runner, char *markdown, cmark_node_type expectedType, char *expectedAttributes) {

src/blocks.c

@@ -1416,7 +1416,7 @@ static void add_text_to_container(cmark_parser *parser, cmark_node *container,
         container = finalize(parser, container);
         assert(parser->current != NULL);
       }
-    } else if (parser->blank) {
+    } else if (parser->blank && (parser->options & CMARK_OPT_PRESERVE_WHITESPACE) == 0) {
       // ??? do nothing
     } else if (accepts_lines(S_type(container))) {
       if (S_type(container) == CMARK_NODE_HEADING &&