Initiating empty swiper throws an exception

[istaiti]

Check that this is really a bug

• I confirm

Reproduction link

I'll add a coverage test

Bug description

up till version 7.0.9, we could init an empty swiper, and later-on add slides
(very helpful in lazy load cases were slides take ages to load)

Expected Behavior

to be able to init an empty swiper

Actual Behavior

console shows a thrown exception, and the init function breaks

Swiper version

33c411e

Platform/Target and Browser Versions

chrome:latest

Validations

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.
• Make sure this is a Swiper issue and not a framework-specific issue

Would you like to open a PR for this bug?

• I'm willing to open a PR