Small formatting fixes

[charlietangora]

Hi!

I tried to use IncludeToolbox with some projects I work on this week and ran into a couple small formatting issues, mostly to do with odd setup.

• If the project includes come in on separate lines with whitespace at the beginning, IsPathRooted throws
• If the preprocessorDefintions string is "", an extra "-D" is emitted into the tmp file, causing iwyu to error

Both were easy fixes and so I am sending in a pull request. (And I am now happily using the tool!)