Allow start/end checks on empty strings

[jvrsantacruz]

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 these corner cases when the string is empty seems like a good idea.

[puppet-community-rangefinder]

stdlib::end_with is a function

that may have no external impact to Forge modules.

stdlib::start_with is a function

that may have no external impact to Forge modules.

This module is declared in 319 of 575 indexed public Puppetfiles.

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[DavidS]

Thanks for your contribution!