empty quotes break pattern replacements in bash-4.2

[Mike Frysinger]

this simple code no longer works in bash-4.2:
$ f=abc; echo ${f##""a}
abc
same goes for ${f//""a} and ${f%%""c}, and perhaps more operations

removing the quotes, or quoting the single char in question, makes it work:
$ f=abc; echo ${f##a} ${f##"a"}
bc bc

the original bug report uses variables in the pattern and quotes them to avoid 
expansion of globs and such.  but if the variable happened to be empty, things 
no longer worked correctly.
-mike

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