Fix leading whitespace in SQL blocks

[joshtemple]

In the LookML editor, SQL statements have a leading space (you can see the highlighted blue background). However, this is probably confusing for users, as they wouldn't think to insert a leading space when they write or modify the SQL statements, leading to confusion when the LookML is generated without one.

We could manage this automatically in the DictParser class by checking for the presence of leading whitespace and inserting a space if none is found.