An Unset Proxy Variable Can Remove the Whole Argument in Fish

In Fish, this diagnostic can print just a newline:

echo HTTPS_PROXY=$HTTPS_PROXY

Fish variables contain lists. When an unquoted variable is attached to text, Fish combines that text with each list element. An undefined variable, or a variable defined with no elements, produces no combinations. The entire argument disappears, including HTTPS_PROXY=.

Keep the label by quoting the expansion:

printf '%s\n' "HTTPS_PROXY=$HTTPS_PROXY"

This supplies one argument even when the variable has no elements, so the label remains visible. Quoting a multi-element variable joins its elements with spaces; it does not prove that the variable contains one usable proxy URL.

To check whether the variable exists, use Fish's query operation:

if set --query HTTPS_PROXY
    printf '%s\n' "HTTPS_PROXY=$HTTPS_PROXY"
else
    echo 'HTTPS_PROXY is not defined'
end

A successful query means the variable is defined, not necessarily that it contains a nonempty value or that a client will use it. Exporting and client-specific proxy discovery are separate checks.