[sql_lab]Disabled run query button if sql query editor is empty (#4728)

* Disabled run query button if sql query editor is empty

* Removing unnecessary white space

* Fix failing test for sql props

* Adding sql variable into propTypes and defaultProps

superset/assets/spec/javascripts/explore/components/RunQueryActionButton_spec.jsx

@@ -16,6 +16,7 @@ describe('RunQueryActionButton', () => {
     runQuery: () => {}, // eslint-disable-line
     selectedText: null,
     stopQuery: () => {}, // eslint-disable-line
+    sql: '',
   };
 
   beforeEach(() => {