[Table] Remove requirement for a header or data row template

It's valid that tables may not want any header, or a table could *only* be a header. Support this use case by letting users avoid defining a `cdk-header-row` or `cdk-row` which means that they should not be inserted.

Motivation: A table footer row could be a single row composed of a single row without a header. If placed after a preceding table, it acts as a sticky footer. Works well for cases where the header should not affect the rows, e.g. a totals row

Note that this doesn't help in cases where a scrollbar appears on one container but not the other. This will cause misalignment on browsers that push content left to show a scrollbar rather than overlay it (e.g. safari)

Empieza buscando en la implementación de tablas y las pruebas de Angular CDK cómo se procesan las plantillas `cdk-header-row` y `cdk-row`. Verifica el comportamiento de una tabla sin encabezado, una tabla sin plantilla de fila de datos y una tabla que solo tenga encabezado; se considera terminado cuando las plantillas omitidas no se insertan, mientras que las filas compatibles siguen renderizándose.

Escrito por el modelo de indexación a partir del texto del issue.