You can detect and replace the missing values:
In[1]:= SetDirectory[NotebookDirectory[]]
Out[1]= "C:\\Users\\David\\Documents\\temp"
In[2]:= raw = Import["missings.xlsx"][[1]]
Out[2]= {{"a", 11., 12., 13.}, {"b", 21., "", 23.}, {"c", "", 32.,
33.}, {"d", 41., 42., 43.}}
In[3]:= data = Replace[raw, "" -> 0, {2}]
Out[3]= {{"a", 11., 12., 13.}, {"b", 21., 0, 23.}, {"c", 0, 32.,
33.}, {"d", 41., 42., 43.}}
But that does not necessarily ignore them. What that means depends on what you are trying to do. For example, you might want the mean of all the elements in a column after nonnumerical values are deleted, so they do not affect the mean.
In[4]:= (* take the mean after removing all nonnumerics *)
mean[list_] := Mean[Select[list, NumberQ]]
In[5]:= (* mean of column 3 *)
mean[raw[[All, 3]]]
Out[5]= 28.6667
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