Fix issue with tink template update using stdin

[detiber]

Description

Fixes an issue with tink template update using stdin, follow up to #587

Why is this needed

#587 didn't fully add support for sdin with tink template update, it still threw an error if content wasn't provided using the --path argument.

How Has This Been Tested?

Tested manually against a Tinkerbell environment created using HEAD of sandbox

How are existing users impacted? What migration steps/scripts do we need?

Existing users not impacted

[detiber]

@mmlb a quick update for #587 that I noticed after finishing up manual testing

[codecov]

Codecov Report

Merging #588 (7d18954) into main (03a3d54) will increase coverage by 0.04%.
The diff coverage is 0.00%.

❗ Current head 7d18954 differs from pull request most recent head f0988a2. Consider uploading reports for the commit f0988a2 to get more accurate results

@@            Coverage Diff             @@
##             main     #588      +/-   ##
==========================================
+ Coverage   38.44%   38.48%   +0.04%     
==========================================
  Files          53       53              
  Lines        3569     3565       -4     
==========================================
  Hits         1372     1372              
+ Misses       2100     2096       -4     
  Partials       97       97              

Impacted Files	Coverage Δ
cmd/tink-cli/cmd/template/update.go	0.00% <0.00%> (ø)

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[mmlb]

Thanks for the fix!