Fix network-form ui

[Sunghee2]

When creating new network settings, Block Explorer URL is optional.
If I write a letter and then erase all letter on this form(Block Explorer URL), save button must be activated because there is no value.
But When I do this, the button is disabled, as shown in the picture.
So I solved this problem by adding checking the value of this form.

[Gudahtt]

Thanks for the contribution!

This solution seems pretty good, except that as a consequence it now means that a blank RPC URL would be allowed too (because it uses the same validateUrl helper function).

Maybe you could make a separate helper function for validating the blockExplorerUrl field. It looks like validateUrl never should have been used anyway - the error case assumes it's an RPC URL (e.g. see on line 221, it'd give the error invalidRPC even if it was an invalid Block Explorer URL)

[Sunghee2]

Thanks you!
As you said, I created a new ValidateBlockExplorerUrl function.

[Gudahtt]

Thanks, looks good!

It would have been better if we could refactor the common steps between the two validation functions, and we should use a better error than invalidRPC, but this is still better than it was before. We can make these other two improvements another time.