[Minor] Test & fix has_only_html_part regression

src/libmime/mime_expressions.c

 
 	PTR_ARRAY_FOREACH(MESSAGE_FIELD(task, text_parts), i, p)
 	{
-		p = g_ptr_array_index(MESSAGE_FIELD(task, text_parts), 0);
-
 		if (!IS_TEXT_PART_ATTACHMENT(p)) {
 			if (IS_TEXT_PART_HTML(p)) {
 				cnt_html++;

test/functional/cases/001_merged/100_general.robot

   ...  Settings={symbols_enabled = [MANY_INVISIBLE_PARTS, ZERO_FONT]}
   Expect Symbol  MANY_INVISIBLE_PARTS
   Expect Symbol  ZERO_FONT
+
+HTML ONLY - TRUE POSITIVE
+  Scan File  ${RSPAMD_TESTDIR}/messages/zerofont.eml
+  ...  Settings={symbols_enabled = [MIME_HTML_ONLY]}
+  Expect Symbol  MIME_HTML_ONLY
+
+HTML ONLY - TRUE NEGATIVE
+  Scan File  ${RSPAMD_TESTDIR}/messages/btc.eml
+  ...  Settings={symbols_enabled = [MIME_HTML_ONLY]}
+  Do Not Expect Symbol  MIME_HTML_ONLY