fix(Datagrid): show column headers for empty infinite scroll

[matthewgallo]

Closes #5240

This PR displays the column headers for empty virtualized/infinite scroll Datagrids. The column headers typically are rendered as part of <DatagridVirtualBody /> because there is special sizing that is required but when there is no data this presents a problem because the headers are not displayed. This renders them normally if we have a withVirtualScroll Datagrid that is empty.

What did you change?

packages/ibm-products-styles/src/__tests__/__snapshots__/styles.test.js.snap
packages/ibm-products-styles/src/components/Datagrid/styles/_datagrid.scss
packages/ibm-products/src/components/Datagrid/Datagrid/DatagridContent.js

How did you test and verify your work?

Storybook

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