Fix empty markdown render request.

app/assets/javascripts/vue_shared/components/markdown/field.vue

@@ -47,22 +47,29 @@
       toggleMarkdownPreview() {
         this.previewMarkdown = !this.previewMarkdown;
 
-        if (!this.previewMarkdown) {
-          this.markdownPreview = '';
-        } else {
-          this.markdownPreviewLoading = true;
-          this.$http.post(
-            this.markdownPreviewPath,
-            {
         /*
           Can't use `$refs` as the component is technically in the parent component
           so we access the VNode & then get the element
         */
-              text: this.$slots.textarea[0].elm.value,
-            },
-          )
+        const text = this.$slots.textarea[0].elm.value;
+
+        if (!this.previewMarkdown) {
+          this.markdownPreview = '';
+        } else {
+          if (text) {
+            this.markdownPreviewLoading = true;
+            this.$http.post(this.markdownPreviewPath, { text })
               .then(resp => resp.json())
               .then((data) => {
+                this.renderMarkdown(data);
+              })
+              .catch(() => new Flash('Error loading markdown preview'));
+          } else {
+            this.renderMarkdown();
+          }
+        }
+      },
+      renderMarkdown(data = {}) {
         this.markdownPreviewLoading = false;
         this.markdownPreview = data.body || 'Nothing to preview.';
 
@@ -74,9 +81,6 @@
         this.$nextTick(() => {
           $(this.$refs['markdown-preview']).renderGFM();
         });
-          })
-          .catch(() => new Flash('Error loading markdown preview'));
-        }
       },
     },
     mounted() {