Distinguish more carefully between "null" inline offsets and "0" inline offsets

Summary:
Ref T13513. Currently, when creating an inline by selecting a line range, slightly careless handling leads to an inline with "0" offsets (by passing "undefined" to the server). This causes the block to highlight every line except the last one as fully bright, which is incorrect.

An inline with "0" offsets and an inline with no offsets are different. Be more careful about passing offsets around and rendering them.

Test Plan:

• Used the line numbers to add an inline to lines 4-8 of a change.
• Hovered the inline.
• Saw all four lines marked as "dull"-highlighted (previously: three bright lines, one dull line).

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21252