The following has evaluated to null or missing:
==> content.author  [in template "json://{\"siteId\":\"0029\",\"category\":\"content\",\"tags\":\"pt\",\"name\":\"pt-gaojian2021\"}_en_US" at line 85, column 153]

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Tip: It's the step after the last dot that caused this error, not those before it.
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Tip: If the failing expression is known to legally refer to something that's sometimes null or missing, either specify a default value like myOptionalVar!myDefault, or use [#if myOptionalVar??]when-present[#else]when-missing[/#if]. (These only cover the last step of the expression; to cover the whole expression, use parenthesis: (myOptionalVar.foo)!myDefault, (myOptionalVar.foo)??
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FTL stack trace ("~" means nesting-related):
	- Failed at: #if content.author != ""  [in template "json://{\"siteId\":\"0029\",\"category\":\"content\",\"tags\":\"pt\",\"name\":\"pt-gaojian2021\"}_en_US" at line 85, column 148]
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