Don't marshal text property of links when it's empty

Slack returns an error if the text property is empty, but if it's not
present everything is fine and it just uses the URL as the text of the
link.

block_rich_text.go

@@ -364,7 +364,7 @@ func NewRichTextSectionEmojiElement(name string, skinTone int, style *RichTextSe
 type RichTextSectionLinkElement struct {
 	Type  RichTextSectionElementType `json:"type"`
 	URL   string                     `json:"url"`
-	Text  string                     `json:"text"`
+	Text  string                     `json:"text,omitempty"`
 	Style *RichTextSectionTextStyle  `json:"style,omitempty"`
 }