Fix problem with let using attributes of {} when they should be null

[dpvc]

The current disassembleToken() and assembleToken() methods produce a new token whose attributes are {} when the original had null attributes. This has implications for some tokens where the difference between these are important, as in the mathchar7 function at

MathJax-src/ts/input/tex/ParseMethods.ts

Lines 156 to 165 in ae389de

	export function mathchar7(parser: TexParser, mchar: Token) {
	const def = mchar.attributes || {mathvariant: MATHVARIANT.NORMAL};
	if (parser.stack.env['font']) {
	// @test MathChar7 Single Font
	def['mathvariant'] = parser.stack.env['font'];
	}
	// @test MathChar7 Single, MathChar7 Operator, MathChar7 Multi
	const node = parser.create('token', 'mi', def, mchar.char);
	parser.Push(node);
	}

where line 157 provides a default attribute list when math.attributes is null (but not if it is an empty object).

This leads to the loss of the default attributes when a new control sequence is \let equal to one that uses mathchar7, as in

\let\percent=\%
\percent\%

where the result of \percent will be in italics, while \% will not.

This PR resolves the issue by actually eliminating disassembleToken() and assembleToken() and just using the original token directly in the method at new line 137 (via the same closure that preserves map).

[zorkow]

Looks good. Works for all my tests.