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/parse-tree/pt-pr-code.h
+++ b/libinterp/parse-tree/pt-pr-code.h
@@ -40,7 +40,7 @@
 public:
 
   tree_print_code (std::ostream& os_arg,
-                   const std::string& pfx = std::string (),
+                   const std::string& pfx = "",
                    bool pr_orig_txt = true)
     : os (os_arg), prefix (pfx), nesting (),
       print_original_text (pr_orig_txt),