fix(sentence-wise-importance): never return an empty array

This was result of refactoring code from observable where return was
used inside function which got propogated to a for loop here!

Author: sanjayaksaxena

src/sentence-wise-importance.js

@@ -79,7 +79,7 @@ const sentenceWiseImportance = function ( rdd ) {
       }
 
       // Ignore sentences where we cannot build NGram i.e. sentences shorter than NGram.
-      if ( pos.length < 4 ) return [];
+      if ( pos.length < 4 ) continue; // eslint-disable-line no-continue
       // Determine NGrams;
       for ( let k = 0; k + NGram - 1 < pos.length; k += 1 ) {
         const pos4Gram = pos.slice( k, k + NGram );
@@ -114,7 +114,9 @@ const sentenceWiseImportance = function ( rdd ) {
         } );
     });
     // Normalize weights by dividing them by the max.
-    const max = Math.max( ...sentenceWiseWeights );
+    let max = Math.max( ...sentenceWiseWeights );
+    // Avoid divide by zero situation
+    if ( max === 0 ) max = 1;
 
     return sentenceWiseWeights.map( ( e, i ) => ( { index: i, importance: +( e / max ).toFixed( 4 ) } ) );
   };

test/its-specs.js

@@ -221,7 +221,8 @@ describe( 'its functions for .out()', function () {
     ];
 
     expect( nlp.readDoc( text ).out( its.sentenceWiseImportance ) ).to.deep.equal( rank );
-    expect( nlp.readDoc( 'text' ).out( its.sentenceWiseImportance ) ).to.deep.equal( [] );
+    expect( nlp.readDoc( 'text' ).out( its.sentenceWiseImportance ) ).to.deep.equal( [ { index: 0, importance: 0 } ] );
+    expect( nlp.readDoc( '' ).out( its.sentenceWiseImportance ) ).to.deep.equal( [ { index: 0, importance: 0 } ] );
   } );
 
   it( 'selected entity with its.detail, its.span as.?', function () {