Add compiler optimization to remove px.contains calls with empty strings

[ddelnano]

Summary: Add compiler optimization to remove px.contains calls with empty strings

The majority of our pxl scripts have parameters that default to empty strings. This results in px.contains function calls that will always return the original data set (an empty string "" will always be found in any input string). An example of this can be seen in the following screenshot (the filter arguments highlighted below):

Relevant Issues: Fixes #618

Type of change: /kind feature

Test Plan: Used analyze and explain query options to test the following conditions:

• Verified what the http_data pxl script looks like when there are no arguments supplied without any optimizations -- graphviz_with_empty_contains
• Verified what the http_data pxl script looks like if the px.contains calls here are removed from the script. The optimization should produce the same graph as this case. -- graphviz_without_empty_contains
• Verified the optimization results in the expected query graph and loaded a variety of pxl scripts to make sure queries appeared normal -- latest_graphviz_with_optimization

[ddelnano]

Moved this to a draft since I didn't expect to run into the sanitization issues and bugs with the GCC build. @pixie-io/maintainers would appreciate a review to see if you have any comments in addition to the fixes I need to get a passing build.