Error out immediatly when using if command without args in command list

When using "if" (or while) without args directly on gdb's command line,
you get this:

  (gdb) if
  if/while commands require arguments

When doing the same when entering a command list, you only get an error
when the command is executed, when parse_exp_in_context_1 fails to
evaluate the expression.

  (gdb) define foo
  Type commands for definition of "foo".
  End with a line saying just "end".
  >if
   >end
  >end
  (gdb) foo
  Argument required (expression to compute).

I think it would make more sense to error out when inputting the command
list directly:

  (gdb) define foo
  Type commands for definition of "foo".
  End with a line saying just "end".
  >if
  if/while commands require arguments.

The only required change is to check whether args is an empty string in
build_command_line.

gdb/ChangeLog:

	* cli/cli-script.c (build_command_line): For if/while commands,
	check whether args is empty.

gdb/testsuite/ChangeLog:

	* gdb.base/commands.exp: Call new procedure.
	(define_if_without_arg_test): New procedure.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2017-09-04  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* cli/cli-script.c (build_command_line): For if/while commands,
+	check whether args is empty.
+
 2017-09-04  Simon Marchi  <simon.marchi@ericsson.com>
 
 	* cli/cli-script.h (enum misc_command_type): Move from defs.h.

gdb/cli/cli-script.c

@@ -147,7 +147,8 @@ build_command_line (enum command_control_type type, const char *args)
 {
   struct command_line *cmd;
 
-  if (args == NULL && (type == if_control || type == while_control))
+  if ((args == NULL || *args == '\0')
+      && (type == if_control || type == while_control))
     error (_("if/while commands require arguments."));
   gdb_assert (args != NULL);
 

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2017-09-04  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* gdb.base/commands.exp: Call new procedure.
+	(define_if_without_arg_test): New procedure.
+
 2017-09-04  Pedro Alves  <palves@redhat.com>
 
 	* gdb.base/list-ambiguous.exp: New file.

gdb/testsuite/gdb.base/commands.exp

@@ -1011,6 +1011,21 @@ proc_with_prefix redefine_backtrace_test {} {
     gdb_test "bt" "hibob" "execute bt command"
 }
 
+# Test using "if" and "while" without args when building a command list.
+
+proc define_if_without_arg_test {} {
+    foreach cmd {if while} {
+	set test "define some_command_$cmd"
+	gdb_test_multiple $test $test {
+	    -re "End with"  {
+		pass $test
+	    }
+	}
+
+	gdb_test "$cmd" "if/while commands require arguments." "type $cmd without args"
+    }
+}
+
 # Test an input line split with a continuation character (backslash)
 # while entering a multi-line command (in a secondary prompt).
 
@@ -1076,5 +1091,6 @@ if_commands_test
 error_clears_commands_left
 redefine_hook_test
 backslash_in_multi_line_command_test
+define_if_without_arg_test
 # This one should come last, as it redefines "backtrace".
 redefine_backtrace_test