fix: check if body is null (emacs-lsp#807)

Author: oxidase

dap-mode.el

@@ -790,12 +790,13 @@ will be reversed."
       (dap--send-message
        (dap--make-request "source" (list :sourceReference sourceReference))
        (dap--resp-handler
-        (-lambda ((&hash "body" (&hash "content" content)))
-          (switch-to-buffer (generate-new-buffer sourceReferenceKey))
-          (insert content)
-          (goto-char (point-min))
-          (forward-line (1- line))
-          (forward-char column))
+        (-lambda ((&hash "body" body))
+          (let ((content (if body (gethash "content" body) "")))
+            (switch-to-buffer (generate-new-buffer sourceReferenceKey))
+            (insert content)
+            (goto-char (point-min))
+            (forward-line (1- line))
+            (forward-char column)))
         (lambda (errmsg)
           (message "No source code for %s. Cursor at %s:%s. Error: %s." name line column errmsg)))
        debug-session))))