Allow empty value for SCHEMA_URL

[fanaugen]

Before this change it was not possible to override the SQLTAP_SCHEMA
with a custom value for trying things out in development, because this
script would download the schema from the dist server and overwrite it,
or exit with an error on line 17 if SCHEMA_URL was set to an empty
string.

[fanaugen]

I need this so I can run sqltap in docker-compose locally by mounting a directory where my schema.xml sits (with local modifications) and overriding the default with this:

version: '2'
services:
  fetch:
    image: local_fetch
    ports:
      - 3307:3000
    volumes:
      - ~/dev/config/common:/config
    environment:
      - SQLTAP_SCHEMA=/config/schema.xml
      - SCHEMA_URL=local

↑ local is just a placeholder, it could as well be any other string that does not start with http

[eliten00b]

Why not http://local ? I mean, what would curl download from local ? And if you dont want to download a remote schema, you dont need.