Support null value for custom attributes.

[meng-tian]

Description

This pull request add null value support for custom attributes of web api. Previously, providing null value to custom attribute xyz will get exception Value is not set for attribute code xyz. The exception is caused by using !isset to examine whether a value exists. !isset return true when value is null.

With this pull request, !array_key_exists will be used instead of !isset. This makesnull value also work for custom attributes.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[adragus-inviqa]

Does this PR mean we'll be getting very large API responses, full of null values?

[meng-tian]

@adragus-inviqa no, this PR only modified ServiceInputProcessor so it should not affect responses.