fix(php): fix php serializeToJsonString empty ListValue.

[lightjiao]

PHP function serializeToJsonString when have an empty ListValue, it return an invalid json, just like this

bug

{
    "name": "jack",
    "friends":,  // here should have an empty `[]`
    "age": 22
}

after fix

{
    "name": "jack",
    "friends": [],
    "age": 22
}

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[lightjiao]

I signed it!

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[googlebot]

CLAs look good, thanks!

[lightjiao]

Hi,
Is there any problem on this pr ?

[TeBoring]

Could you check whether master branch has already fixed this problem?

[TeBoring]

#5106 may have fixed this.

[TeBoring]

If it's not fixed, could you add a test to encode_decode_test.php?

[lightjiao]

ok,
thanks.

[lightjiao]

I have test on master branch, the problem has already fixed, but there is no empty ListValue test case in encode_decode_test.php.

I would add a test case.

thank you.