Fix error on contract method proxy when method parameter is null.

[Pedro-vk]

Description

After upgrade to Web3 beta 55, the method proxy was failing because some parameters were null instead of undefined. This change allows receiving null parameters.

Type of change

• Bug fix

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no warnings.
• I have updated or added types for all modules I've changed
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run test in the root folder with success and extended the tests if necessary.
• I ran npm run build in the root folder and tested it in the browser and with node.
• I ran npm run dtslint in the root folder and tested that all my types are correct
• I have tested my code on an ethereum test network.

[coveralls]

Coverage remained the same at 95.947% when pulling b10536a on Pedro-vk:patch-2 into 9528160 on ethereum:1.0.

[coveralls]

Coverage remained the same at 95.947% when pulling f4a37ca on Pedro-vk:patch-2 into ce93e2d on ethereum:1.0.