maint: Replace emtpy 'std::string ()' calls with "". * Backend.cc, comment-list.h, debug.cc, defaults.in.h, dynamic-ld.h, error.cc, graphics.cc, graphics.in.h, help.cc, hook-fcn.h, load-path.cc, load-path.h, ls-ascii-helper.cc, ls-mat-ascii.cc, ls-mat5.cc, ls-mat5.h, ls-oct-text.cc, oct-iostrm.h, oct-lvalue.h, oct-stream.cc, oct-stream.h, oct-strstrm.h, regexp.cc, schur.cc, sqrtm.cc, strfns.cc, symtab.cc, symtab.h, sysdep.cc, toplev.cc, urlwrite.cc, variables.cc, variables.h, ov-builtin.h, ov-cell.cc, ov-class.cc, ov-classdef.cc, ov-classdef.h, ov-dld-fcn.h, ov-fcn-handle.cc, ov-fcn-inline.cc, ov-fcn-inline.h, ov-fcn.h, ov-lazy-idx.cc, ov-mex-fcn.h, ov-struct.cc, ov-typeinfo.cc, ov-typeinfo.h, ov-usr-fcn.cc, ov-usr-fcn.h, ov.h, ovl.h, octave.cc, lex.h, parse.h, pt-classdef.h, pt-exp.cc, pt-pr-code.h, token.h, version.cc, version.in.h, oct-rand.h, dir-ops.h, file-ops.cc, file-ops.h, file-stat.h, oct-env.cc, oct-group.cc, oct-passwd.cc, oct-syscalls.cc, cmd-edit.cc, cmd-edit.h, cmd-hist.cc, cmd-hist.h, kpse.cc, lo-array-gripes.h, pathsearch.cc, pathsearch.h, str-vec.h, url-transfer.h: Replace emtpy 'std::string ()' calls with "".

--- a/libinterp/corefcn/graphics.in.h
+++ b/libinterp/corefcn/graphics.in.h
@@ -734,7 +734,7 @@
 
   std::string string_value (void) const
   {
-    return value.empty () ? std::string () : value[0];
+    return value.empty () ? "" : value[0];
   }
 
   string_vector string_vector_value (void) const { return value; }
@@ -828,7 +828,7 @@
 class radio_values
 {
 public:
-  OCTINTERP_API radio_values (const std::string& opt_string = std::string ());
+  OCTINTERP_API radio_values (const std::string& opt_string = "");
 
   radio_values (const radio_values& a)
     : default_val (a.default_val), possible_vals (a.possible_vals) { }
@@ -2305,7 +2305,7 @@
 
   static std::string default_toolkit (void)
   {
-    return instance_ok () ? instance->do_default_toolkit () : std::string ();
+    return instance_ok () ? instance->do_default_toolkit () : "";
   }
 
 private: