Adding default properties to NetworkForm

[ryanml]

Fixes #10681

In NetworkForm, the initial is state is given default values of component properties. When navigating to the "Add a network form" from the network dropdown specifically, those initial properties are undefined. In the case of adding a network, those fields should default to ''. This simply adds a defaultProps block ensuring that the values are prop || '' as they should not be undefined when set as the initial value of a text input field.

Manual testing steps:
Defined in issue

Reviewer's manual testing note - a simple regression test for adding and managing custom networks should be performed from both the network dropdown and the settings tab.

[metamaskbot]

Builds ready [992b5ae]

• builds: chrome, firefox, opera
• bundle viz: background, ui, inpage, contentscript, ui-libs, bg-libs, phishing-detect
• build viz: Build System
• code coverage: Report
• storybook: Storybook
• all artifacts

Page Load Metrics (548 ± 22 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	42	60	46	5	2
		domContentLoaded	350	571	546	46	22
		load	351	571	548	46	22
		domInteractive	350	570	546	46	22

[rickycodes]

makes sense to me