use dict.get() to safely access key in Holders

[ericpien]

• using dict.get() is safer than dict[key] and allows effective way to return non-None variable
• Fixes Error: Failed to parse holders json data. #1940:

Sample Checks:

ticker = yf.Ticker('AACG') # from the attached major_holders.not.found_0-1.txt
print(ticker.major_holders)

Breakdown                        Value
insidersPercentHeld            0.07085
institutionsPercentHeld        0.16265
institutionsFloatPercentHeld   0.17506
institutionsCount             12.00000

ticker = yf.Ticker('RVSN') # from the attached major_holders.not.found_4-5.txt
print(ticker.major_holders)

Breakdown                        Value
insidersPercentHeld            0.38820
institutionsPercentHeld        0.02128
institutionsFloatPercentHeld   0.03478
institutionsCount             12.00000