when sdrPreserve is missing (or admin is missing) for a file, assume TRUE

[ndushay]

PR #609 makes it so we only generate tech md for Files when they explicitly have sdrPreserve set to true.

Not sure if changing the default in cocina models (issue sul-dlss/cocina-models#87) will be sufficient to ensure that this service will always generate techmd; it depends if this service always receives fully fleshed out file objects, or can get the abbreviated ones as in the tests added in PR #609.

If this service can get abbreviated File model objects (without default explicit) then the behavior here will need to change.

[ndushay]

#609 dealth with this.