Use an explicit placeholder for missing manifest thumbnails (#3447)

src/components/ManifestListItem.js

@@ -104,6 +104,8 @@ export class ManifestListItem extends React.Component {
               >
                 <Grid container spacing={2} className={classes.label} component="span">
                   <Grid item xs={4} sm={3} component="span">
+                    { thumbnail
+                      ? (
                         <Img
                           className={[classes.thumbnail, ns('manifest-list-item-thumb')].join(' ')}
                           src={[thumbnail]}
@@ -119,6 +121,8 @@ export class ManifestListItem extends React.Component {
                             />
                           )}
                         />
+                      )
+                      : <Skeleton className={classes.placeholder} variant="rect" height={80} width={120} />}
                   </Grid>
                   <Grid item xs={8} sm={9} component="span">
                     { isCollection && (
@@ -139,6 +143,8 @@ export class ManifestListItem extends React.Component {
             </Grid>
 
             <Grid item xs={4} sm={2}>
+              { manifestLogo
+                && (
                 <Img
                   src={[manifestLogo]}
                   alt=""
@@ -154,6 +160,7 @@ export class ManifestListItem extends React.Component {
                     />
                   )}
                 />
+                )}
             </Grid>
           </Grid>
         ) : (