Error handling for empty vellum data

[Thejas-bhat]

• Handling this case will help in avoiding any downstream empty slice panics that might occur.
• An example with respect to this panic is as follows:

panic: runtime error: index out of range [0] with length 0   
goroutine 1537342 [running]: github.com/blevesearch/vellum.(*fstStateV1).isEncodedSingle(...) 
/home/couchbase/.cbdepscache/gomodcache/pkg/mod/github.com/blevesearch/vellum@v1.0.8/decoder_v1.go:100 github.com/blevesearch/vellum.(*fstStateV1).TransitionFor(0xc014e04de0?, 0x40?) 
/home/couchbase/.cbdepscache/gomodcache/pkg/mod/github.com/blevesearch/vellum@v1.0.8/decoder_v1.go:239 +0x2d2 github.com/blevesearch/vellum.(*FST).get(0xc014e11cc0, {0xc01c7db558, 0x5, 0xc010531a68?}, {0x15cac98, 0xc01cb846e8}) 
/home/couchbase/.cbdepscache/gomodcache/pkg/mod/github.com/blevesearch/vellum@v1.0.8/fst.go:78 +0xd0 github.com/blevesearch/vellum.(*Reader).Get(...) 
/home/couchbase/.cbdepscache/gomodcache/pkg/mod/github.com/blevesearch/vellum@v1.0.8/fst.go:299 github.com/blevesearch/zapx/v15.(*Dictionary).postingsList(0xc01cb7cba0, {0xc01c7db558?, 0x1?, 0xc01358a500?}, 0xc0129bdd70?, 0xc01cb7b7c0) 
/home/couchbase/.cbdepscache/gomodcache/pkg/mod/github.com/blevesearch/zapx/v15@v15.3.4/dict.go:57 +0x88 github.com/blevesearch/zapx/v15.

This happens when the backend fst which is the term dictionary is loaded with an empty byte slice of data from the index file, and when we try to do a Get() on the fst, we would end up doing operations on the empty slice that causes a panic.

[abhinavdangeti]

@Thejas-bhat Would you add some sample panics to the commit message^, for our reference.