Currently, `tool_app_template::run_async()` crashes when invoked with empty argv (with just `argv[0]` populated). This can happen if the tool app is invoked without any further args, e.g. just invoking `scylla nodetool`. The crash happens because unconditional dereferencing of `argv[1]` to get the current operation.
To fix, add an early-exit for this case, just printing a usage message and exiting with exit code 2.
Fixes: #16451Closesscylladb/scylladb#16456
* github.com:scylladb/scylladb:
test: add regression tests for invoking tools with no args
tools/utils: tool_app_template: handle the case of no args
tools/utils: tool_app_template: remove "scylla-" prefix from app name