Handling case when there are no tests in testable summary

[timbaev]

When running the command, we would get an error:

java.lang.NullPointerException
	at io.eroshenkoam.xcresults.export.ExportCommand.lambda$runUnsafe$1(ExportCommand.java:120)
	at java.base/java.util.HashMap.forEach(HashMap.java:1339)
	at io.eroshenkoam.xcresults.export.ExportCommand.runUnsafe(ExportCommand.java:115)
	at io.eroshenkoam.xcresults.export.ExportCommand.run(ExportCommand.java:86)
	at picocli.CommandLine.executeUserObject(CommandLine.java:1769)
	at picocli.CommandLine.access$900(CommandLine.java:145)
	at picocli.CommandLine$RunLast.handle(CommandLine.java:2141)
	at picocli.CommandLine$RunLast.handle(CommandLine.java:2108)
	at picocli.CommandLine$AbstractParseResultHandler.execute(CommandLine.java:1975)
	at picocli.CommandLine.execute(CommandLine.java:1904)
	at io.eroshenkoam.xcresults.XCResults.main(XCResults.java:13)

After debugging it turned out that there was an empty XCTestCase without tests in the application target, so the export failed.
Added a check for the presence of tests, if they are not, then display a message about it in the log.

Perhaps this partially closes the issue #33