2.19.x: Exception thrown when a disabled check is passed to -XepPatchChecks

[AlexLandau]

Standalone repro: https://github.com/AlexLandau/errorprone-disabled-and-patch-issue

We have some odd situations involving automated tooling where sometimes, we will run patch creation on checks that have also been disabled; that is, we end up with both -Xep:CheckName:OFF and -XepPatchChecks:CheckName. In 2.18.0, this resulted in the patch being created as if it weren't disabled. Starting in 2.19.0 (and through the current version, 2.19.1), an exception is thrown instead:

/home/alandau/code/gradle-disabled-and-patch-issue/lib/src/main/java/com/github/alexlandau/repro/Library.java:5: error: An unhandled exception was thrown by the Error Prone static analysis plugin.
    public String toString() {
                  ^
     Please report this at https://github.com/google/error-prone/issues/new and include the following:

     error-prone version: 2.19.1
     BugPattern: MissingOverride
     Stack Trace:
     java.util.NoSuchElementException: No value present
        at java.base/java.util.Optional.get(Optional.java:143)
        at com.google.errorprone.matchers.Description.severity(Description.java:78)
        at com.google.errorprone.ErrorProneAnalyzer.lambda$finished$1(ErrorProneAnalyzer.java:138)
        at com.google.errorprone.VisitorState.reportMatch(VisitorState.java:301)
        at com.google.errorprone.scanner.Scanner.reportMatch(Scanner.java:127)
        at com.google.errorprone.scanner.ErrorProneScanner.processMatchers(ErrorProneScanner.java:448)
        at com.google.errorprone.scanner.ErrorProneScanner.visitMethod(ErrorProneScanner.java:739)
        (etc., full stacktrace in repo linked above)

We can work around this for the time being (editing the -Xep args to make sure the check isn't disabled).