In https://github.com/LibreDWG/libredwg/issues/346 I described roundtripping issue in conversion between JSON and DWG.

In the conversion from DWG to JSON is used:
```
DWG_TABLE (LTYPE)
...
FIELD_BINARY (strings_area, 256, 0);
```
in dwg.spec
The result of the conversion of "" is 512 character string with zeros.

In the conversion from JSON to DWG is used:
```
in_hex2bin() ...
```
in in_dxf.c
The result of the conversion of 512 character string with zeros is "0".

I am curious when we have in_hex2bin without tests, and there is a copy of this function in unit-testing/add_test.c.

I think that could be fine to have one function and tests with it. + Fix of this issue.

DWG to JSON and back strings_area issue
This is a GitHub Issue
repo:libredwg
owner:LibreDWG
Title : DWG to JSON and back strings_area issue
Issue date:

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In https://github.com/LibreDWG/libredwg/issues/346 I described roundtripping issue in conversion between JSON and DWG.

In the conversion from DWG to JSON is used:
```
DWG_TABLE (LTYPE)
...
FIELD_BINARY (strings_area, 256, 0);
```
in dwg.spec
The result of the conversion of "" is 512 character string with zeros.

In the conversion from JSON to DWG is used:
```
in_hex2bin() ...
```
in in_dxf.c
The result of the conversion of 512 character string with zeros is "0".

I am curious when we have in_hex2bin without tests, and there is a copy of this function in unit-testing/add_test.c.

I think that could be fine to have one function and tests with it. + Fix of this issue.

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None