function "dtutils.string.substitute()" fails when no time data is provided in Exif data

先在 lua/lib/dtutils/string.lua 中阅读 build_substitute_list 和 exiftime2systime，然后针对没有 EXIF 时间数据的图像运行所报告的替换脚本。在受影响的系统上默认时间戳能够被接受且不出现 os.time 错误，即表示完成。

When running a script using "dtutils.string.substitute()" function on a file without any time data in Exif, the "dtutils.string.substitute()" function fails.

190.9832 LUA ERROR : ...\AppData\Local\darktable/lua/lib/dtutils\string.lua:748: time result cannot be represented in this installation
stack traceback:
	[C]: in ?
	[C]: in function 'os.time'
	...AppData\Local\darktable/lua/lib/dtutils\string.lua:748: in upvalue 'exiftime2systime'
	...AppData\Local\darktable/lua/lib/dtutils\string.lua:843: in function 'lib/dtutils.string.build_substitute_list'
	...AppData\Local\darktable/lua/lib/dtutils\string.lua:1298: in function 'lib/dtutils.string.substitute'

After looking at the "string.lua" code, it seems that "build_substitute_list" function tries to use "0000:00:00 00:00:00" as default value if the exif time is not available, but the "os.time{}" conversion called later through "exiftime2systime(datetime_taken)" does not like it. The minimum time value accepted on several systems is January 1st, 1970. I updated the "string.lua" code on my installation (replacing "0000:00:00 00:00:00" with "1970:01:01 00:00:00") and this fixes the issue.

Code from string.lua :

local function exiftime2systime(exiftime)
  local yr,mo,dy,h,m,s = string.match(exiftime, "(%d-):(%d-):(%d-) (%d-):(%d-):(%d+)")
  return(os.time{year=yr, month=mo, day=dy, hour=h, min=m, sec=s})
end

[...]

function dtutils_string.build_substitute_list(image, sequence, variable_string, username, pic_folder, home, desktop)
[...]
  if image.exif_datetime_taken and image.exif_datetime_taken ~= "" then
    datetime_taken = image.exif_datetime_taken
  else
    if use_millisecs then
      datetime_taken = "0000:00:00 00:00:00.0"
    else
      datetime_taken = "0000:00:00 00:00:00"
    end
  end
[...]

I see this issue on Windows 11, DarkTable 5.4.0. Other systems could behave the same, or not.