Fix infinite loop on EOF in the command line debugger

When using the command line debugger (godot -d) on Unix systems, when
entering an EOF (ctrl+D), the debugger enters an infinite loop.

Adding a check for EOF in the debugger loop exits the debugger when EOF
is entered.

Fixes godotengine#50170.

core/debugger/local_debugger.cpp

@@ -138,7 +138,7 @@ void LocalDebugger::debug(bool p_can_continue, bool p_is_error_breakpoint) {
 		// Cache options
 		String variable_prefix = options["variable_prefix"];
 
-		if (line.is_empty()) {
+		if (line.is_empty() && !feof(stdin)) {
 			print_line("\nDebugger Break, Reason: '" + script_lang->debug_get_error() + "'");
 			print_line("*Frame " + itos(current_frame) + " - " + script_lang->debug_get_stack_level_source(current_frame) + ":" + itos(script_lang->debug_get_stack_level_line(current_frame)) + " in function '" + script_lang->debug_get_stack_level_function(current_frame) + "'");
 			print_line("Enter \"help\" for assistance.");
@@ -267,7 +267,8 @@ void LocalDebugger::debug(bool p_can_continue, bool p_is_error_breakpoint) {
 				print_line("Added breakpoint at " + source + ":" + itos(linenr));
 			}
 
-		} else if (line == "q" || line == "quit") {
+		} else if (line == "q" || line == "quit" ||
+				(line.is_empty() && feof(stdin))) {
 			// Do not stop again on quit
 			script_debugger->clear_breakpoints();
 			script_debugger->set_depth(-1);