kicad-cli: image file name is empty on simple symbols
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# Description
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After 8c3eeb5201384f1ff22059d086de736ad2f9a60a, basic symbols (no demorgan, single unit) export to ".svg", no file name. If they have multiple units, the generated file name is correct (abcdefg_1.svg, etc).
# Steps to reproduce
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1. Prep a library with simple symbols and complex symbols
2. `kicad-cli sym export svg yourlib.kicad_sym`
3. Note that all the simple symbols get plotted to `.svg`.
# KiCad Version
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```
Application: KiCad x86_64 on x86_64
Version: 7.0.5-4d25ed1034~172~ubuntu22.04.1, release build
Libraries:
wxWidgets 3.2.1
FreeType 2.11.1
HarfBuzz 6.0.0
FontConfig 2.13.1
libcurl/7.81.0 OpenSSL/3.0.2 zlib/1.2.11 brotli/1.0.9 zstd/1.4.8 libidn2/2.3.2 libpsl/0.21.0 (+libidn2/2.3.2) libssh/0.9.6/openssl/zlib nghttp2/1.43.0 librtmp/2.3 OpenLDAP/2.5.14
Platform: Ubuntu 22.04.2 LTS, 64 bit, Little endian, wxGTK, xubuntu, x11
Build Info:
Date: May 27 2023 23:08:46
wxWidgets: 3.2.1 (wchar_t,wx containers) GTK+ 3.24
Boost: 1.74.0
OCC: 7.5.2
Curl: 7.88.1
ngspice: 38
Compiler: GCC 11.3.0 with C++ ABI 1016
Build settings:
KICAD_SPICE=ON
```
issue