PLOT: fixed "file not read" warn. for empty files

Bug was caused when the respective data file was empty, thus no data was
added to the gnuplot scripts, and gnuplot threw a "file could not be
read" - warning
Fix: Empty data files will cause a zero-line to be added to the gnuplot
script.
PLOT: fixed "multiple wildcards"-warning

Bug: A warning should be thrown if there are 2 or more wildcards in one
mathematical expression of a custom plot. The former check also counted
wildcards used in the naming section of the expression, resulting in a
false warning.
Fix: Counts only wildcards in the actual expression section.
PLOT: fixed non showing functions

Bug: If a plot contained a mathematical expression with wildcard, every
functinon containing a '*' would be removed, due to a
wildcard-replacement attempt on said function.
Fix: Wildcard replacement skips functions as it should.
PLOT: fixed bug witth Type not add to customplots