`toStringWithSourceMap` behaves incorrectly if `SourceNode`s source is `null` or is an empty string

When source set to null, mapping does not generated.
When source set to empty string there is an error during mapping calculation (not during SourceNode creation as expected if empty string is forbidden).

This code snippet demonstrates both problems: https://runkit.com/embed/47lduu6k2173

Source code of the snippet:

let SourceNode = require('source-map').SourceNode;

function test(source) {
  let node = new SourceNode(
    null,
    null,
    source,
    [
      'prefix',
      new SourceNode(1, 0, source, 'code'),
      'suffix',
    ]
  );
  let { code, map } = node.toStringWithSourceMap();
  console.log(map.toJSON(), code);
}
// {version: 3, sources: ["source"], names: [], mappings: "MAAA,I"}
test("source");// OK
// {version: 3, sources: [], names: [], mappings: ""}
test(null);    // Empty mapping
// Error: Invalid mapping: {"generated":{"line":1,"column":6},"source":"","original":{"line":1,"column":0},"name":null}
test("");      // Error

The entry point is SourceNode.toStringWithSourceMap; first reproduce the null and empty-string cases from the issue snippet and trace how each source reaches mapping generation. Done means both inputs have intentional, consistent behavior without silently losing mappings or failing later with an invalid-mapping error.