Throw error if identifier sending function is given empty identifiers

[dwinter]

As shown in #107, when using functions like entrez_link, entrez_fetch and entrez_summary with programmatically derived ID vectors it is possible to accidentally send empty ids.

There is no good reason to do this, and it can produce hard-to-debug errors. We should instead catch these before they are sent to the NCBI and provide a useful error message.