gdb/mi: handle no condition argument case for -break-condition

As reported in PR gdb/28076 [1], passing no condition argument to the
-break-condition command (e.g.: "-break-condition 2") should clear the
condition for breakpoint 2, just like CLI's "condition 2", but instead
an error message is returned:

  ^error,msg="-break-condition: Missing the <number> and/or <expr> argument"

The current implementation of the -break-condition command's argument
handling (79aabb7308 "gdb/mi: add a '--force' flag to the
'-break-condition' command") was done according to the documentation,
where the condition argument seemed mandatory.  However, the
-break-condition command originally (i.e. before the 79aabb7308
patch) used the CLI's "cond" command, and back then not passing a
condition argument was clearing out the condition.  So, this is a
regression in terms of the behavior.

Fix the argument handling of the -break-condition command to allow not
having a condition argument, and also update the document to make the
behavior clear.  Also add test cases to test the scenarios which were
previously not covered.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=28076

gdb/doc/gdb.texinfo

@@ -30565,7 +30565,7 @@ times="0"@}
 @subsubheading Synopsis
 
 @smallexample
- -break-condition [ --force ] @var{number} @var{expr}
+ -break-condition [ --force ] @var{number} [ @var{expr} ]
 @end smallexample
 
 Breakpoint @var{number} will stop the program only if the condition in
@@ -30573,7 +30573,8 @@ Breakpoint @var{number} will stop the program only if the condition in
 @samp{-break-list} output (see the description of the @samp{-break-list}
 command below).  If the @samp{--force} flag is passed, the condition
 is forcibly defined even when it is invalid for all locations of
-breakpoint @var{number}.
+breakpoint @var{number}.  If the @var{expr} argument is omitted,
+breakpoint @var{number} becomes unconditional.
 
 @subsubheading @value{GDBN} Command
 

gdb/mi/mi-cmd-break.c

@@ -423,20 +423,19 @@ mi_cmd_break_condition (const char *command, char **argv, int argc)
 	}
     }
 
-  /* There must be at least two more args: a bpnum and a condition
-     expression.  */
-  if (oind + 1 >= argc)
-    error (_("-break-condition: Missing the <number> and/or <expr> "
-	     "argument"));
+  /* There must be at least one more arg: a bpnum.  */
+  if (oind >= argc)
+    error (_("-break-condition: Missing the <number> argument"));
 
   int bpnum = atoi (argv[oind]);
 
   /* The rest form the condition expr.  */
-  std::string expr (argv[oind + 1]);
-  for (int i = oind + 2; i < argc; ++i)
+  std::string expr = "";
+  for (int i = oind + 1; i < argc; ++i)
     {
-      expr += " ";
       expr += argv[i];
+      if (i + 1 < argc)
+	expr += " ";
     }
 
   set_breakpoint_condition (bpnum, expr.c_str (), 0 /* from_tty */,

gdb/testsuite/gdb.mi/mi-break.exp

@@ -435,6 +435,28 @@ proc_with_prefix test_forced_conditions {} {
     mi_gdb_test "-break-info 16" \
 	"\\^done,[mi_make_breakpoint_table [list $bp]]" \
 	"invalid condition is defined"
+
+    # No cond argument should clear the condition.
+    mi_gdb_test "-break-condition 16" \
+	"~\"Breakpoint 16's condition is now valid at location 1, enabling.*\\^done" \
+	"clear the condition"
+    set bp [mi_make_breakpoint -number 16]
+    mi_gdb_test "-break-info 16" \
+	"\\^done,[mi_make_breakpoint_table [list $bp]]" \
+	"condition is cleared"
+
+    # Zero-argument is an error.
+    mi_gdb_test "-break-condition" \
+	"\\^error,msg=\"-break-condition: Missing the <number> argument\"" \
+	"no arguments to -break-condition"
+
+    # Passing --force with no condition should not crash or raise an error.
+    mi_gdb_test "-break-condition --force 16" \
+	"\\^done" \
+	"clear the condition with --force"
+    mi_gdb_test "-break-condition --force" \
+	"\\^error,msg=\"-break-condition: Missing the <number> argument\"" \
+	"no arguments with --force"
 }
 
 proc test_break {mi_mode} {