Implement blank_none option, satisfying #35

[smt5541]

🛠 Changes being made

Give examples of the changes you've made in this pull request. Include an itemized list if you can.

• Added FPV_BLANK_NONE configuration option, which is handled as False if not set
• Added blank_none parameter to Parameter constructor, which defaults to FPV_BLANK_NONE if not set, determines if an empty string passed to an Optional[str] parameter will be converted to None
• Rewrote handling of Optional, as implementing blank_none broke the existing implementation

🧠 Rationale behind the change

• The change was requested by @summersz

Why did you choose to make these changes?

• These changes were requested in issue Conversion of empty string to None #35, and developers familiar with certain other languages and frameworks may be used to receiving null in place of an empty string, so this option could help those developers use this library more smoothly

Does this pull request resolve any open issues?

Closes #35

Were there any trade-offs you had to consider?

🧪 Testing

• [ X ] Have tests been added or updated for the changes introduced in this pull request?

• [ X ] Are the changes backwards compatible?

If the changes aren't backwards compatible, what other options were explored?

✨ Quality check

• [ X ] Are your changes free of any erroneous print statements, debuggers or other leftover code?

• [ X ] Has the README been updated to reflect the changes introduced (if applicable)?

💬 Additional comments

[Ge0rg3]

Looks great 😄