inconsistent behavior of LOAD_FILE depending on whether dolt sql-server is running.

[nicktobey]

When running dolt sql without connecting to an existing engine, Dolt will refuse to load files that exist outside of the repo:

> echo Hello world > test
> dolt sql -q "select LOAD_FILE('test');
+-------------------+
| LOAD_FILE('test') |
+-------------------+
| Hello world       |
|                   |
+-------------------+

> cp test ..
> dolt sql -q "select LOAD_FILE('../test');"
+----------------------+
| LOAD_FILE('../test') |
+----------------------+
| NULL                 |
+----------------------+

However, if the server is running in a separate process, the load is successful:

dolt sql-server &
dolt sql -q "select LOAD_FILE('../test');"
+----------------------+
| LOAD_FILE('../test') |
+----------------------+
| Hello world          |
|                      |
+----------------------+

I'm not sure which behavior is preferred, but we should aim to be consistent. (And if the intent is to block the file read, we might want to record a warning instead of just having LOAD_FILE return NULL.