illustration of a peculiarity of TeX paragraph building

figure.tex

\documentclass{article}

\usepackage{mwe}
\begin{document}

% or anything filling the line with less than inter-word space shrinkability
\noindent\rule{\linewidth}{1pt} % <-- we deliberately leave a space token here
\begin{figure}[htbp]
  \centering
  % only by way of example, close to real life examples
  \includegraphics[height=1cm]{example-image-a}
  \caption{Hello, is there extra space above figure env?}
\end{figure}

\noindent\rule{\linewidth}{1pt}

\begin{figure}[htbp]
  \centering
  \includegraphics[height=1cm]{example-image-a}
  \caption{This one doesn't have any...}
\end{figure}
\end{document}

for example with

AAAAAAAA  AAAAAAAA  AAAAAAAA  AAAAAAAA  AAAAAAAA IIII.
\begin{figure}[htbp]
  \centering
  x
  \caption{Hello, is there extra space above figure env?}
\end{figure} {} {}

and the modified sphack, there is no issue.