[gdb/symtab] Handle empty file name in .debug_line section

With DWARF 5, it's possible to produce an empty file name in the File Name
Table of the .debug_line section:
...
 The File Name Table (offset 0x112, lines 1, columns 2):
  Entry Dir     Name
  0     1       (indirect line string, offset: 0x2d):
...

Currently, when gdb reads an exec containing such debug info, it segfaults:
...
Thread 1 "gdb" received signal SIGSEGV, Segmentation fault.
0x000000000072cd38 in dwarf2_start_subfile (cu=0x2badc50, fe=..., lh=...) at \
  gdb/dwarf2/read.c:18716
18716     if (!IS_ABSOLUTE_PATH (filename) && dirname != NULL)
...
because read_direct_string transforms "" into a nullptr, and we end up
dereferencing the nullptr.

Note that the behaviour of read_direct_string has been present since repo
creation.

Fix this in read_formatted_entries, by transforming nullptr filenames in to ""
filenames.

Tested on x86_64-linux.

Reviewed-By: Tom Tromey <tom@tromey.com>

PR symtab/30357
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30357

gdb/dwarf2/line-header.c

@@ -215,6 +215,10 @@ read_formatted_entries (dwarf2_per_objfile *per_objfile, bfd *abfd,
 	      break;
 	    }
 
+	  /* Normalize nullptr string.  */
+	  if (string.has_value () && *string == nullptr)
+	    string.emplace ("");
+
 	  switch (content_type)
 	    {
 	    case DW_LNCT_path:

gdb/testsuite/gdb.dwarf2/dw2-empty-file-name.exp

@@ -0,0 +1,66 @@
+# Copyright 2023 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Check that an empty file name in the .debug_line section doesn't cause
+# problems.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+require dwarf2_support
+
+# The .c files use __attribute__.
+require is_c_compiler_gcc
+
+standard_testfile main.c -dw.S
+
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    declare_labels Llines
+    global srcdir subdir srcfile
+
+    cu { version 5 } {
+	compile_unit {
+	    {language @DW_LANG_C}
+	    {name $srcfile}
+	    {stmt_list $Llines DW_FORM_sec_offset}
+	} {
+	    subprogram {
+		{external 1 flag}
+		{MACRO_AT_func {main}}
+	    }
+	}
+    }
+
+    lines {version 5} Llines {
+	include_dir "${srcdir}/${subdir}"
+	file_name "" 1
+    }
+}
+
+if { [build_executable "failed to prepare" ${testfile} \
+	  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+clean_restart
+
+# If there's an ERROR during gdb_load, the assert will produce an UNRESOLVED
+# rather than a FAIL.
+gdb_assert { [gdb_load $binfile] == 0 }
+
+# But this will produce an actual FAIL, which is more visible than
+# ERROR/UNRESOLVED.
+gdb_assert { [info exists gdb_spawn_id] }