Crash when opening empty file

[Mikhail Lopatkin]

java.lang.NullPointerException
    at com.google.common.base.CharMatcher.trimFrom(CharMatcher.java:1195)
    at org.bitbucket.mlopatkin.android.liblogcat.file.FileDataSourceFactory.getFirstNonEmptyLine(FileDataSourceFactory.java:43)
    at org.bitbucket.mlopatkin.android.liblogcat.file.FileDataSourceFactory.createDataSource(FileDataSourceFactory.java:52)
    at org.bitbucket.mlopatkin.android.logviewer.MainFrame$11.actionPerformed(MainFrame.java:429)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)

[Mikhail Lopatkin]

Use CharSource instead of files to improve FDSF testability

Ref #129, #120

→ <<cset 5ab78fa3b6cf>>

[Mikhail Lopatkin]

Fix #120: Stop crashing when opening empty file

Now with tests, hope there'll be no more regressions.

→ <<cset 9a5d7b57f7e2>>