Add link to language guides for empty query results

Author: eysk-io

extensions/ql-vscode/src/view/raw-results-table.tsx

@@ -27,14 +27,19 @@ export class RawTable extends React.Component<RawTableProps, {}> {
       dataRows = dataRows.slice(0, RAW_RESULTS_LIMIT);
     }
 
-    const tableRows = dataRows.map((row: ResultRow, rowIndex: number) =>
-      <RawTableRow
-        key={rowIndex}
-        rowIndex={rowIndex + this.props.offset}
-        row={row}
-        databaseUri={databaseUri}
-      />
-    );
+    const emptyTableRow = <tr key={0}><td key={-1}>
+      This query returned no results. If this isn't what you're expecting, and for tips writing effective queries, check out the <a href="https://codeql.github.com/docs/codeql-language-guides/">CodeQL language guides</a>
+    </td></tr>
+
+    const tableRows = dataRows.length > 0 ?
+      dataRows.map((row: ResultRow, rowIndex: number) =>
+        <RawTableRow
+          key={rowIndex}
+          rowIndex={rowIndex + this.props.offset}
+          row={row}
+          databaseUri={databaseUri}
+        />
+      ) : [emptyTableRow];
 
     if (numTruncatedResults > 0) {
       const colSpan = dataRows[0].length + 1; // one row for each data column, plus index column