Fix exception when calling LayerGroup/hasLayer() with wrong layerId

[johnd0e]

Now false returned, as per docs
fix #6472

[johnd0e]

Note:
Currently undefined/false/null are tolerated as layer value.

There is even special code in tests to ensure that:

Leaflet/spec/suites/layer/LayerGroupSpec.js

Lines 2 to 9 in cd12b94

	describe("#hasLayer", function () {
	it("returns false when passed undefined, null, or false", function () {
	var lg = L.layerGroup();
	expect(lg.hasLayer(undefined)).to.equal(false);
	expect(lg.hasLayer(null)).to.equal(false);
	expect(lg.hasLayer(false)).to.equal(false);
	});
	});

In my opinion it has little sense but can lead to bugs.
Could anyone explain source reason?
Edit: #6999

[IvanSanchez]

LGTM

[IvanSanchez]

Note:
Currently undefined/false/null are tolerated as layer value.
[...]
In my opinion it has little sense but can lead to bugs.
Could anyone explain source reason?

That dates back to... wow, #4. My guess is that at some point, the meaning (and documentation) of hasLayer() changed without anybody noticing.

[IvanSanchez]

The travis CI tests are failing with

  77:7   error  Identifier 'layer_id' is not in camel case  camelcase
  78:10  error  Identifier 'layer_id' is not in camel case  camelcase

[johnd0e]

The travis CI tests are failing with

Fixed