Empty blobs as bind parameters being returned as NULLs

[msfstef]

Currently empty blobs can be written and read unless they are inserted as bind parameters, in which case they get returned as null. Issue seems to be that nothing gets allocated for them and are interpreted as NULLs.

PR with suggested fix: #167

[sgbeal]

That's how the C library works: https://sqlite.org/c3ref/column_blob.html returns a C NULL for length-zero blobs, and C NULL is interpreted as SQL NULL except in cases where a C NULL is interpreted as an allocation error.

[rhashimoto]

There's additional relevant discussion in the PR. Summary: this behavior by the C library is less than ideal but altering that behavior in a wrapper API is also not ideal. There's no everyone wins solution here, unfortunately.