Add optional check and fix button deprecated parameters

[ilyasfoo]

Changes proposed in this Pull Request:

Closes #1525.

• Silences JS error by adding optional check
• Fixes deprecated parameter for button component by replacing it with variant parameter

How to test the changes in this Pull Request:

1. Ensure your site has no published product (optionally, trash them temporarily)
2. Open JS console
3. Go to Homescreen
4. Complete Customize your store task if it's not yet completed
5. Observe product task header loads correctly (title, description, CTA button)
6. Observe no undefined error in JS console

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.

[github-actions]

Size Change: +18 B (+0.01%)

Total Size: 201 kB

Filename	Size	Change
./build/index.js	126 kB	+18 B (+0.01%)

ℹ️ View Unchanged

Filename	Size
./build/53.js	1.08 kB
./build/index.css	731 B
./build/marketing.js	58.3 kB
./build/payment-gateway-suggestions.css	1.24 kB
./build/payment-gateway-suggestions.js	6.57 kB
./build/plugins.js	3.93 kB
./build/style-index.css	2.15 kB
./build/style-marketing.css	800 B

_{compressed-size-action}

[chihsuan]

LGTM and tested well! 🚢