If you fail to specify an Import-Package at all, then your bundle cannot access the package even if it available and exported by some other bundle.
If you import the package optionally, then your bundle can use it if available, but that package not being available does not prevent resolution of your bundle.
You should avoid optional dependencies at the package level. Either you depend on a package or you don't. Normally a need for optional dependencies indicates that your bundle has poor coherency, i.e. it's doing too many unrelated things within one module.