Put Zero address if no from address supplied

[arajasek]

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Description

If the API's estimateGas function is called without a from address, the VM uses the wrong offset when serialising. The fix is to simply use the Zero Address instead.

Fixes Issue #642

Type of change

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• Bug fix.
• New feature.
• Enhancement.
• Unit test.
• Breaking change (a fix or feature that causes existing functionality to not work as expected).
• Requires documentation update.

Testing

All existing tests pass

Verification

Insert x into the following checkboxes to confirm (eg. [x]):

• I have self-reviewed my own code and conformed to the style guidelines of this project.
• New and existing tests pass locally with my changes.
• I have added tests for my fix or feature.
• I have made appropriate changes to the corresponding documentation.
• My code generates no new warnings.
• Any dependent changes have been made.