Allow JLine to fall back to a dumb terminal

[mbovel]

Set the dumb JLine option to null instead of false when a dumb terminal is not forced. This allows JLine to fall back to a dumb terminal if needed.

Fixes #11978.

Before:

➜ ~/dotty git:(mb/11978) ✗ echo 'object Example { def smth = "smth" }' | bin/scala
The `--offline` option is experimental
Please bear in mind that non-ideal user experience should be expected.
If you encounter any bugs or have feedback to share, make sure to reach out to the maintenance team at https://github.com/VirtusLab/scala-cli
Exception in thread "main" java.lang.IllegalStateException: Unable to create a terminal
        at org.jline.terminal.TerminalBuilder.doBuild(TerminalBuilder.java:394)
        at org.jline.terminal.TerminalBuilder.build(TerminalBuilder.java:362)
        at dotty.tools.repl.JLineTerminal.<init>(JLineTerminal.scala:34)
        at dotty.tools.repl.ReplDriver.runUntilQuit(ReplDriver.scala:145)
        at dotty.tools.repl.ReplDriver.tryRunning(ReplDriver.scala:136)
        at dotty.tools.repl.Main$.main(Main.scala:7)
        at dotty.tools.repl.Main.main(Main.scala)
        Suppressed: java.io.IOException: Unable to find terminal provider ffm
                at org.jline.terminal.spi.TerminalProvider.load(TerminalProvider.java:74)
                at org.jline.terminal.TerminalBuilder.checkProvider(TerminalBuilder.java:667)
                at org.jline.terminal.TerminalBuilder.getProviders(TerminalBuilder.java:632)
                at org.jline.terminal.TerminalBuilder.doBuild(TerminalBuilder.java:395)
                ... 6 more
        Suppressed: java.io.IOException: Unable to find terminal provider jni
                at org.jline.terminal.spi.TerminalProvider.load(TerminalProvider.java:74)
                at org.jline.terminal.TerminalBuilder.checkProvider(TerminalBuilder.java:667)
                at org.jline.terminal.TerminalBuilder.getProviders(TerminalBuilder.java:634)
                at org.jline.terminal.TerminalBuilder.doBuild(TerminalBuilder.java:395)
                ... 6 more
        Suppressed: java.io.IOException: Unable to find terminal provider jansi
                at org.jline.terminal.spi.TerminalProvider.load(TerminalProvider.java:74)
                at org.jline.terminal.TerminalBuilder.checkProvider(TerminalBuilder.java:667)
                at org.jline.terminal.TerminalBuilder.getProviders(TerminalBuilder.java:636)
                at org.jline.terminal.TerminalBuilder.doBuild(TerminalBuilder.java:395)
                ... 6 more

After:

➜  ~/dotty git:(mb/11978) echo 'object Example { def smth = "smth" }' | bin/scala
The `--offline` option is experimental
Please bear in mind that non-ideal user experience should be expected.
If you encounter any bugs or have feedback to share, make sure to reach out to the maintenance team at https://github.com/VirtusLab/scala-cli
Aug 05, 2024 3:53:49 PM org.jline.utils.Log logr
WARNING: Unable to create a system terminal, creating a dumb terminal (enable debug logging for more information)
Welcome to Scala 3.6.0-RC1-bin-SNAPSHOT-git-dce4377 (17.0.8, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> // defined object Example

scala> %                                                                                                                         
➜  ~/dotty 

[mbovel]

@smarter: that's an implementation of what you suggested in #11707 (comment).