Fix \figure{} output from md when alternate text is empty

Then we need to leave out the second \figure{} argument,
according to WRE. This fixes pkgdown on roxygen2.

Closes #1051.

NEWS.md

@@ -11,6 +11,10 @@
 * roxygen2 now adds hyperlinks to R6 methods in the PDF manual as well,
   (#1006).
 
+* roxygen2 now omits empty annotations (alternate text) for figures added
+  via markdown. This caused issues when generating pkgdown web sites
+  (#1051).
+
 # roxygen2 7.0.2
 
 * `\example{}` escaping has been improved (again!) so that special escapes 

R/markdown.R

@@ -348,7 +348,10 @@ mdxml_link_text <- function(xml_contents, state) {
 mdxml_image = function(xml) {
   dest <- xml_attr(xml, "destination")
   title <- xml_attr(xml, "title")
-  paste0("\\figure{", dest, "}{", title, "}")
+  paste0(
+    "\\figure{", dest, "}",
+    if (nchar(title)) paste0("{", title, "}")
+  )
 }
 
 escape_comment <- function(x) {