[11.x] fix BigDecimal creation with blank value on HasAttributes.php

[dukmaurice]

Issue

Currently you can't cast attribute of decimal type with an empty string

// Return Illuminate\Support\Exceptions\MathException: Unable to cast value to a decimal.
BigDecimal::of(" ")->toScale($decimals, RoundingMode::HALF_UP);

Proposal

I propose to check value passed for BigDecimal creation for prevent MathException

// Before
BigDecimal::of($value)->toScale($decimals, RoundingMode::HALF_UP);

// After
BigDecimal::of(blank($value) ? 0 : $value)->toScale($decimals, RoundingMode::HALF_UP);

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!