Splitting text can leave |start| and |end| Positions without renderers

When establishing new start and end positions in ApplyStyleCommand::applyInlineStyle()
be sure to return early if either of them end up null, just as we do if either of
them are null initially.

In this test case execCommand('CreateLink') adds an anchor HTML element in an SVG
namespace so the text underneath validly does not receive a renderer. Without a renderer
the text won't get a visible position value for the command so there's nothing to do but
bail.

BUG=339185,397076

Review URL: https://codereview.chromium.org/651303003

git-svn-id: svn://svn.chromium.org/blink/trunk@184124 bbb929c8-8fbe-4397-9dbb-9b2b20218538