For those who are interested, I've finally solved my problem (the vertical
space before a subsection which was not automatically swallowed at the top
of a page when the page started with an empty section) by using the pair
'after={\vskip 0em\blank[disable]}' for sections:


\setuphead
  [section]
  [placehead=empty,
   page=yes,
   after={\vskip 0em\blank[disable]}]

\setuphead
  [subsection]
  [placehead=yes,
   before={\blank[line]},
   after={\blank[line]}]

\setupheadertexts[section]

\showframe

\starttext
   \completecontent
   \dorecurse{2}
     {\section{My section}
      \dorecurse{2}
        {\subsection{My subsection}
         \input knuth}}
\stoptext