org-export-as: Fix subtree export when metadata ends right before heading

* lisp/ox.el (org-export-as): Only include first newline after heading
line when the metadata is non-empty.
* testing/lisp/test-ox.el (test-org-export/export-scope): Add test.

Reported-by: Kaushal Modi <kaushal.modi@gmail.com>
Link: https://orgmode.org/list/CAFyQvY1d=UEJ-6ZPG1X+st=fCfBAnTfgbVNhxyxH_xKcLGG4cQ@mail.gmail.com

lisp/ox.el

@@ -3040,7 +3040,7 @@ Return code as a string."
                ;; This way, we will be able to retrieve its export
                ;; options when calling
                ;; `org-export--get-subtree-options'.
-               (backward-char)
+               (when (bolp) (backward-char))
 	       (narrow-to-region (point) (point-max))))
         ;; Initialize communication channel with original buffer
         ;; attributes, unavailable in its copy.

testing/lisp/test-ox.el

@@ -1113,6 +1113,16 @@ Text"
 	    (org-export-as (org-test-default-backend)
 			   'subtree nil nil
 			   '(:with-planning t :with-properties t)))))
+  (should
+   (equal ""
+	  (org-test-with-temp-text "
+* H
+:PROPERTIES:
+:A: 1
+:END:<point>
+* H2"
+	    (org-export-as (org-test-default-backend)
+			   'subtree))))
   ;; Visible.
   (should
    (equal "* H1\n"