refactor(material/snack-bar): fix out-of-sync code

[crisbeto]

Fixes some code that is out of sync with the code we have internally. I added the extra @if when I was resolving issues with #25273.

[devversion]

LGTM to bring this in sync. OOC: Is this expected though? The individual theming system functions should not be invoked with null, guarded by the theme mixins checking for null

[crisbeto]

This came up during the presubmit where some apps were ending up with a partial/non-existent typography config. I did it this way to maintain backwards compatibility.

[devversion]

Sounds reasonable. Generally this is unexpected though and we shouldn't need these checks elsewhere, except for e.g. backwards compatibility here (in which case we may want to clean this up in the future I think)

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