Allow start/end checks on empty strings

When using this library function on strings that come from a parameter,
they might be empty sometimes, rendering it difficult to use, having to
force hacks like.

"_${source}".stdlib::start_with('_puppet:///modules/')

Where _ is added to make sure that the string is never empty.

In Ruby the start_with? function allows being used on empty strings,
and it returns false for any assertion to be done:

p ''.start_with?('test')
false

Also in Python the startswith() method returns false for empty
strings:

In [1]: print(''.startswith('test'))
False

As this logic seems the most extended and allows for a wider use of the
library, removing corner cases when the string comes empty.