Plot: Exception with 0 image and sqrt colormap
Maybe not the last silx version.
```
ERROR:silx.gui.qt._qt:<class 'AssertionError'>    File "/users/valls/Software/miniconda3/envs/blissenv/lib/python3.7/site-packages/silx/gui/_glutils/OpenGLWidget.py", line 224, in paintGL
    parent.paintGL()
  File "/users/valls/Software/miniconda3/envs/blissenv/lib/python3.7/site-packages/silx/gui/plot/backends/BackendOpenGL.py", line 413, in paintGL
    self._paintFBOGL()
  File "/users/valls/Software/miniconda3/envs/blissenv/lib/python3.7/site-packages/silx/gui/plot/backends/BackendOpenGL.py", line 363, in _paintFBOGL
    self._renderPlotAreaGL()
  File "/users/valls/Software/miniconda3/envs/blissenv/lib/python3.7/site-packages/silx/gui/plot/backends/BackendOpenGL.py", line 678, in _renderPlotAreaGL
    self._renderItems(overlay=False)
  File "/users/valls/Software/miniconda3/envs/blissenv/lib/python3.7/site-packages/silx/gui/plot/backends/BackendOpenGL.py", line 454, in _renderItems
    isXLog, isYLog)
  File "/users/valls/Software/miniconda3/envs/blissenv/lib/python3.7/site-packages/silx/gui/plot/backends/glutils/GLPlotImage.py", line 468, in render
    self._renderLinear(matrix)
  File "/users/valls/Software/miniconda3/envs/blissenv/lib/python3.7/site-packages/silx/gui/plot/backends/glutils/GLPlotImage.py", line 397, in _renderLinear
    self._setCMap(prog)
  File "/users/valls/Software/miniconda3/envs/blissenv/lib/python3.7/site-packages/silx/gui/plot/backends/glutils/GLPlotImage.py", line 340, in _setCMap
    dataMin, dataMax = self.cmapRange  # If log, it is stricly positive
  File "/users/valls/Software/miniconda3/envs/blissenv/lib/python3.7/site-packages/silx/gui/plot/backends/glutils/GLPlotImage.py", line 286, in cmapRange
    assert self._cmapRange[0] >= 0. and self._cmapRange[1] > 0.
```
diff --git a/silx/gui/plot/backends/glutils/GLPlotImage.py b/silx/gui/plot/backends/glutils/GLPlotImage.py
index cfb57c6c0..fb672fa69 100644
--- a/silx/gui/plot/backends/glutils/GLPlotImage.py
+++ b/silx/gui/plot/backends/glutils/GLPlotImage.py
@@ -291,7 +291,7 @@ class GLPlotColormap(_GLPlotData2D):
         if self.normalization == 'log':
             assert self._cmapRange[0] > 0. and self._cmapRange[1] > 0.
         elif self.normalization == 'sqrt':
-            assert self._cmapRange[0] >= 0. and self._cmapRange[1] > 0.
+            assert self._cmapRange[0] >= 0. and self._cmapRange[1] >= 0.
         return self._cmapRange
 
     @cmapRange.setter
['silx/gui/plot/backends/glutils/GLPlotImage.py']