Avoid StringIndexOutOfBoundsException

Some users are reporting that they see a "String index out of range: -1" message when opening certain projects in the STS Roo shell. This message appears to be caused by a StringIndexOutOfBoundsException that gets thrown from PomManagementServiceImpl#getModuleForFileIdentifier when the given project has no pom.xml file. This patch will allow getModuleForFileIdentifier to return null when no pom.xml file can be found.