Add assertions to ExportNamedDeclaration without from

[sosukesuzuki]

Q	A
Fixed Issues?	N/A
Patch: Bug Fix?	Y
Tests Added + Pass?	Yes
License	MIT

Add assertions to ExportNamedDeclaration without from when importAssertions is enabled.

According to estree spec:

assertions must be an empty array when source is null.

[babel-bot]

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[nicolo-ribaudo]

Ok, it looks like it doesn't support it 😬

[JLHwung]

We can assign assertions immediately after we have parsed them:

babel/packages/babel-parser/src/parser/statement.js

Line 2019 in 966387d

if (expect) {

[sosukesuzuki]

Actually, node.source = null is called in maybeParseExportNamedSpecifiers and maybeParseExportDeclaration. So we can remove this from here.