Parsing Unicode string with nulls in R - r

Question

I am having some trouble parsing a Unicode string a JSON object pulled from an API. As the string has Encoding() like "unknown", i need to parse it for the system to know what its dealing with. The string represents a decoded .png file in UTF-8 that I then need to decode back to latin1 before writing it to a .png file (I know, very backwards, and it would be much better if the API pushed a base64-string).
I get the string from the API as chr object, and try to let fromJSON do the job, but no dice. It cuts the string at the first null (\u0000).
> library(httr)
> library(jsonlite)
> library(tidyverse)
> m
Response [https://...]
Date: 2018-04-10 11:47
Status: 200
Content-Type: application/json
Size: 24.3 kB
{"artifact": "\u0089PNG\r\n\u001a\n\u0000\u0000\u0000\rIHDR\u0000\u0000\u0000\u0092\u0000\u0000\u0000\u00e3...
> x <- content(m, encoding = "UTF-8", as = "text")
> ## substing of the complete x:
> x <- "{\"artifact\": \"\\u0089PNG\\r\\n\\u001a\\n\\u0000\\u0000\\u0000\\rIHDR\\u0000\\u0000\\u0000\\u0092\\u0000\\u0000\\u0000\\u00e3\\b\\u0006\\u0000\\u0000\\u0000n\\u0005M\\u00ea\\u0000\\u0000\\u0000\\u0006bKGD\\u0000\\u00ff\\u0000\\u00ff\\u0000\\u00ff\\u00a0\\u00bd\\u00a7\\u0093\\u0000\\u0000\\u0016\\u00e7IDATx\\u009c\\u00ed\"}\n"
>
> ## the problem
> "\u0000"
Error: nul character not allowed (line 1)
> ## this works fine
> "\\u0000"
[1] "\\u0000"
>
> y <- fromJSON(txt = x)
> y # note how the string is cut!
$artifact
[1] "\u0089PNG\r\n\032\n"
When I replace the \\u0000 with char(0), everything works fine. The problem is that the nulls seems to play an important role in the binary representation of the file that I write to in the end, causing the resulting image to be corrupted in the viewer.
> x <- str_replace_all(string = x, pattern = "\\\\u0000", replacement = chr(0))
> y <- fromJSON(txt = x)
> y
$artifact
[1] "\u0089PNG\r\n\032\n\rIHDR\u0092ã\b\006n\005Mê\006bKGDÿÿÿ ½§\u0093\026çIDATx\u009cí"
> str(y$artifact)
chr "<U+0089>PNG\r\n\032\n\rIHDR<U+0092>ã\b\006n\005Mê\006bKGDÿÿÿ ½§<U+0093>\026çIDATx<U+009C>í"
> Encoding(y$artifact)
[1] "UTF-8"
> z <- iconv(y$artifact, from = "UTF-8", to = "latin1")
> writeBin(object = z, con = "test.png", useBytes = TRUE)
I have tried these commands with the original string, to no avail
> library(stringi)
> stri_unescape_unicode(str = x)
Error in stri_unescape_unicode(str = x) :
embedded nul in string: '{"artifact": "<U+0089>PNG\r\n\032\n'
> ## and
> parse(text = x)
Error in parse(text = x) : nul character not allowed (line 1)
Is there no way for R to handle this nul character?
Any idea on how I can get the complete encoded string and write it to a file?
The same story works just fine in Python, which uses a \x convention in stead of \u00
response = r.json()
artifact = response['artifact']
artifact
'\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR....'
artifact_encoded = artifact.encode('latin-1')
artifact_encoded # note the binary form!
b'\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR....'
fh = open("abc.png", "wb")
fh.write(artifact_encoded)
fh.close()
FYI: I have cut some most of the actual string out, but enough to use for testing purposes. The actual string contained other symbols, and it seemed impossible to copy-paste the string in a script and assign it to a new variable (e.g. y <- "{\"artifact\": \"\\u0089PNG\\..."). So, I don't know what I would do if I had to read the string from e.g. a .csv file..
Any pointers in any of my struggles would be appreciated :)