Add check for source, remove unnecessary null return

Co-authored-by: Deniz Kusefoglu <deniz@influxdata.com>
Co-authored-by: Daniel Campbell <metalwhirlwind@gmail.com>

ui/src/sources/components/SourceStep.tsx

@@ -162,10 +162,9 @@ class SourceStep extends PureComponent<Props, State> {
 
   private get passwordPlaceholder() {
     const {source} = this.props
-    if (source.authentication === 'basic') {
+    if (source && source.authentication === 'basic') {
       return 'Value saved in server'
     }
-    return null
   }
 
   private get authIndicator(): JSX.Element {